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Article

Research on the Heating Effect of a Convection Radiator Based on a Human Thermophysiological Model

by
Zongjiang Liu
1,2,
Wei Xu
1,2,*,
Linhua Zhang
1,
Zhong Li
2 and
Ji Li
2
1
School of Thermal Engineering, Shandong Jianzhu University, Jinan 250101, China
2
Institute of Building Environment and Energy, China Academy of Building Research Co., Ltd., Beijing 100013, China
*
Author to whom correspondence should be addressed.
Buildings 2024, 14(1), 199; https://doi.org/10.3390/buildings14010199
Submission received: 13 December 2023 / Revised: 8 January 2024 / Accepted: 10 January 2024 / Published: 12 January 2024
(This article belongs to the Section Building Energy, Physics, Environment, and Systems)
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Abstract

:
Forced convection is the most effective way to improve the thermal efficiency of a radiator under low-temperature conditions. This technical method differs significantly from the heating effects of general radiation and natural convection. Few studies have applied the objective evaluation method based on quantitative calculation to evaluate the effectiveness of indoor heating or optimize the technical parameters (air flow rate, air supply method, etc.) of heating systems. This article couples human metabolic factors with heating environmental factors and uses a 57-node human thermal physiological model to evaluate the effectiveness of forced convection radiator heating from the perspective of the local thermal comfort of the human body and demonstrates the feasibility of this scheme by comparing it with floor radiation heating. The research shows that the air supply speed of a radiator affects human thermal comfort. Continuing to increase the wind speed, at a speed of 3 m/s, the surface temperature of the human body reaches a high value and will then decrease, leading to a decrease in thermal comfort. Research on indoor air distribution shows that the use of bottom-side air supply provides better thermal comfort compared to top air supply. The local skin temperature distribution of the human body indicates that when the indoor average temperature is higher than 20 °C, the overall thermal comfort of convective radiator heating and floor radiant heating is comparable. The article provides a way of objectively calculating and directly quantifying the effect of heating equipment on human thermal physiological parameters.

1. Introduction

With the development of the economy and the advancement of science and technology, the human demand for comfort in building environments is gradually increasing. Thermal comfort has always been an important indicator for evaluating indoor environments and is also one of the most basic needs for human survival. The indicators used to evaluate environmental thermal comfort include the effective temperature index (ET), the standard effective temperature index (SET*), and the predicted mean vote (PMV) index for predicting thermal comfort [1]. Among them, the PMV is the most representative indicator for predicting thermal comfort, which includes four environmental factors, namely, the dry bulb temperature of air, the partial pressure of water vapor, air flow rate, and the radiation temperature of indoor objects and walls, and two factors related to individuals themselves, namely, metabolic rate and clothing thermal resistance [2]. The thermal physiological comfort of people in indoor environments depends on three factors [3]: environmental factors, factors related to human physiological characteristics, and factors related to clothing/textiles. The complexity of the PMV thermal comfort model establishment process and the impact of individual differences and the adaptability of experimenters on subjective judgments can become a source of bias in predicting environmental thermal comfort [4]. One of the future research directions is to introduce more objective evaluation indicators into the PMV thermal comfort model or establish a thermal comfort model that is less dependent on human perception and thus subjective.
The human body has a self-regulation function for various thermal environments, and it adapts to new environments through a series of complex physiological processes. In response to these findings, thermal physiological models have been developed. The most influential multi-node model for human thermal physiology is the 25-node temperature regulation model developed by Stolwijk [5]. This model was developed for the National Aeronautics and Space Administration (NASA) during the Apollo program to create a mathematical model that could predict the thermal response of astronauts during space activities outside the spacecraft. The thermophysiological model includes two parts: a passive system and an active system. Passive systems simulate the physical state of the human body, simulate heat transfer between internal structures and between the human body and the environment [6], and perform internal heat transfer and body heat transfer calculations. The thermal characteristics of blood, muscles, fat, and bones are key parameters [7]. The body exchanges heat it generates with heat in the environment through conduction, convection, radiation, and evaporation [8]. The passive system is controlled by the human body’s active system to maintain a constant body temperature when affected by changing environmental parameters. The active system simulates the regulation of blood vessel contraction, vasodilation, tremors, and sweating in the human body and regulates blood vessel contraction and expansion through changes in brain temperature signals [9], thereby regulating body heat.
In recent years, significant advances have been made in the development of advanced thermoregulation models [10,11]. In order to depict the detailed temperature distribution throughout the entire human body, an advanced three-dimensional (3D) thermoregulation model was developed. Kang et al. [12] developed an advanced 3D thermoregulation model using computational fluid dynamics (CFD) tools that can replace human experiments to circumvent ethical approval requirements and operational limitations in terms of exposure and efficiency. Researchers typically rely on the coupling of human body temperature regulation models with thermal human body models to simulate human thermal physiological responses under given environmental conditions and with regard to the type of clothing worn [13,14]. In the field of applied research on human thermal physiological models, Schellen [15] discussed the use of thermophysiological models to evaluate thermal sensation in building environments, and the results confirmed that when local effects (local skin temperature and thermal sensation) have a significant impact, the PMV cannot predict systemic thermal sensation. Van Hoof et al. [16] conducted an extensive literature review on the effectiveness of PMV models, indicating that for more complex thermal environmental conditions (such as transient or non-uniform), empirical models (such as the PMV index) may become less suitable. Yamamoto et al. [17] investigated the influence of convective heat-transfer coefficient variations on thermophysiology and their contribution to the PMV calculation values using a coupled analysis method of CFD and energy consumption simulation tools. Joshi et al. [18] used ANSYS Fluent (2020 R1) to establish a three-dimensional solid model and accurately predicted human thermal physiological responses under various environmental conditions. Awais et al. [19] used the commercial software THESEUS FE 6.1 (to simulate the human thermal physiological response of subjects wearing tight sportswear. In the THESEUS finite element analysis program, the heat dissipation model was coupled with the Fiala temperature regulation model [20]. Sevilgen and Kilic [21] used CFD techniques to study thermal comfort and water transfer in a room with two heating radiators. Teixeira et al. [22] coupled CFD code with a multi-node thermoregulatory model to determine body skin temperature via two methods: constant temperature and different temperature measurements in different local parts. Angelova et al. [23] analyzed the thermal physiological comfort of operating room patients and surgeons and simulated a surgical clothing combination with three clothing insulation values. Thermophysiological models are used in many fields. In biometeorology, they were used to develop a universal climate index (UTCI) [6] to analyze and evaluate human thermal response under different clothing thermal resistance [24,25]. By combining thermophysiological models with thermal human models, a thermophysiological human simulator for clothing comfort analysis was developed [26]. Such models have also been developed to predict the human thermal response to design stadiums and buildings [27], to predict the thermophysiological response of anesthetized patients during surgery [28], and to evaluate the thermal comfort of drivers during cab driving [5]. The above various studies indicate that the human thermal physiological model provides a scientific tool for studying the thermal comfort of the human body under non-uniform and transient conditions.
However, few studies have applied the thermophysiological model to evaluate the effectiveness of indoor heating (cooling) or optimize the technical parameters (air flow rate, water supply temperature, air supply method, etc.) of heating ventilation and air conditioning (HVAC) systems. This study utilizes the 57-node human thermal physiological effect model integrated with STAR CCM+ [29] to study the heating effect of convective-heating radiators. The innovative research in this paper mainly lies in three aspects: First, this article couples human metabolic factors with environmental factors for simulation calculations. This approach is completely different from the current method of using subjective voting to study environmental thermal comfort and provides a direct quantitative evaluation path for determining the local thermal comfort of the human body under the effect of different heating methods based on objective calculations. Second, this study investigates the effects of floor radiation heating and convective radiator heating on the local thermal comfort of the human body, analyzes the key factors that affect human thermal comfort when heated in these two ways, and proposes improvements to the heating method employed in convective radiators (air supply form, temperature, air supply speed, etc.). Finally, the coupling method can obtain practical and reliable predictions of heat transfer between the human body and the environment, which can be directly fed back to the human thermal regulation model, thereby accurately evaluating the thermal sensation and comfort of various parts. This requires further research on the coupling of the two to promote the widespread use of this method.

2. Research Methods

In this study, we applied a multi-node human thermal physiological model to evaluate the heating effect of convective radiators. This article establishes a thermal physiological parameter calculation model for radiator heating based on the human body thermal physiological model. This article refers to the climate laboratory shown in Figure 1 to establish a research model. The Climate Laboratory is located at the Experimental Base of the Chinese Academy of Building Research in Songzhuang Town, Tongzhou District, Beijing, China. There are two sets of air-conditioning systems installed on the indoor and outdoor sides of the laboratory. The outdoor side is a low-temperature, air-conditioned room, simulating a low-temperature outdoor environment. Based on the characteristic parameters of the above laboratory, a CFD calculation model for the climate laboratory was established, as shown in Figure 2. The indoor side has spatial dimensions of 3400 mm (length) × 4000 mm (width) × 2800 mm high, and those of the outdoor side are 3400 mm (length) × 2000 mm (width) × 2800 mm high. The vertical three-sided enclosure structure on the outdoor side corresponds to an adiabatic boundary condition, and the top corresponds to a velocity boundary condition. The air supply temperature was set according to the working conditions; the bottom is the outlet boundary condition, and the air supply speed was 0.5 m/s. The purpose of setting up outdoor side rooms was to create an outdoor environment with a certain temperature and to consider the convective heat transfer characteristics of air supply and partition walls. Figure 2 show the grid details of the CFD computational model.
The details of the computational model established in this article are shown below. Figure 3 shows the main boundary conditions used in the calculation of this article.
In this study, we used the thermal comfort calculation module from the STAR CCM+ platform to establish a dynamic human thermal physiological model that can calculate the thermal physiological parameters of 14 components of the human body. As shown in Figure 4, each component is divided into 4 layers (core, fat, muscle, and skin) and includes a central circulatory system, with a total of 57 nodes in the simulation model. This provides research ideas and tools for studying the heating non-uniformity of heating end devices, the impact of temperature gradients on thermal comfort, and the comprehensive effects of flow and radiation on human thermal comfort. The division of the human body and the simulation model are shown in Figure 4 and Table 1.

3. Verification of CFD Model

Skin temperature is one of the important physiological indicators of the human body. It is closely related to human thermal sensation and can reflect the thermal balance and thermal sensation state of the human body. In addition, many factors related to human thermal comfort affect the thermal balance of the human body by affecting heat exchange between the skin surface and the environment, thereby affecting the thermal sensation of the human body. Zhou Hao and others from Xi’an Jianzhu University conducted tests on human skin temperature in different environments [30] using a total of 56 participants. The designed environmental temperatures were divided into four working conditions: 23 °C, 28 °C, 33 °C, and 38 °C. The basic clothing worn consisted of long pants, a sports top, and sports shoes, with a clothing thermal resistance of 1.1 clo. The author used the regression analysis method via SPSS 19.0 statistical software to conduct a regression analysis on the skin temperatures of various parts of the collected clothing and obtained regression equations for the local skin temperature and average skin temperature at the environmental temperature under different clothing thermal resistances. The regression equations concerning human skin temperature and environmental temperature are listed in Table 2.
When calculating CFD parameters, the thermal resistance of the clothing was set to 1.1 clo. Only the hands and head were exposed, while the rest of the body accounts for the thermal resistance of the clothing. The head, torso, left upper arm, left thigh, left hand, and left foot were selected as the research subjects. The deviation between the experimental regression values and the simulated calculation values at different indoor temperatures was calculated, and the comparison results are shown in Figure 5, Figure 6, Figure 7, Figure 8, Figure 9 and Figure 10.
From the above data, it can be seen that the deviation between the simulated values and experimental regression values of various parts of the body decreases with the increase in room temperature. The data deviation of the head decreases from 8% to 3% with the increase in temperature, and the data deviation of the hands decreases from 11% to 2% with the increase in temperature. The overall deviation of the body data is stable below 3%. The overall deviation of the data for the left upper arm and left thigh remains stable below 2%. The overall deviation of the left-foot data remains stable below 5%. Comparing the ambient temperatures of 21.38 °C and 25.28 °C (the experimental simulation conditions ranged from 23 to 38 °C), the deviation between the calculated values and the regression values decreases when approaching or staying within the experimental regression conditions. This indicates that the CFD human body thermophysiological model has strong temperature adaptability and reliability with respect to predicting human surface temperature. When the indoor ambient temperature rises above 21 °C, the calculated values of the human thermal physiological model are highly consistent with the experimental regression values. Therefore, it can be considered that the human surface temperature calculated using the human thermal physiological analysis model on the STAR CCM+ platform can objectively reflect the actual physiological characteristics of the human body.

4. Results and Discussion

As traditional pieces of heating terminal equipment, the operating conditions of radiators are constantly changing with the changes in the form of heating sources and the reduction in building heat load. In the context of the increasingly widespread application of low-temperature heat sources, improving the thermal performance of radiators under low-temperature conditions is also a hot research topic both domestically and internationally. Adnan Ploskić et al. studied the overall performance of forced convection heat radiators [31]. Research has shown that the use of forced convection radiators in residential buildings can meet the heating requirements of buildings with lower water supply temperatures. The importance of airflow rate and convection plate design to the operational performance of heating radiators equipped with forced convection devices has been explored. Under the condition of supply and return water temperatures of 45 °C and 35 °C, the analyzed radiator can increase the temperature of the incoming airflow from −5 °C to 26 °C at a flow rate of 10 L/s. Ploskic and Holmberg [32] used theoretical methods to analyze the application effect of skirting-ventilation-type radiators in offices. The heat generated by a ventilated radiator system is 2.1 times that of a traditional radiator (with a supply air velocity of 7.0 L/s and an intake air temperature of −6.0 °C). Research has shown that gradually increasing the forced convection intensity of a radiator will compensate for the decrease in heat dissipation caused by gradually decreasing the water supply temperature of the radiator. This type of forced convection radiator differs significantly from ordinary natural convection radiators in terms of its heat transfer mechanism and effect on indoor thermal comfort. We used a human thermal physiological model as an analysis tool to study the heating effect of this piece of heating equipment in order to improve the technical application conditions of this apparatus and provide technical support for the evolution of plate radiators to low-temperature convection-heating radiators. The basic structure of the radiator studied in this article is shown in Figure 11.

4.1. Research on Air Flow Characteristics of Convective Radiator Heating

In this study, we set the thermal resistance of clothing to 0 to study the direct interaction between the indoor air supply flow field and human skin because clothing significantly reduces the comfort perception of the human body towards changes in indoor air flow velocity. The ambient temperature of the climate laboratory in the model was 16 °C, with a convective heat transfer coefficient of 15.0 W/m2K. The outdoor side was under low-temperature conditions, and the top air inlet temperature was −20 °C, with a wind speed of 0.5 m/s. The inlet temperature of the radiator was 80 °C, the inlet flow rate was 7 g/s, and the outlet temperature of the radiator changed with different convective intensities. The temperatures of the human body and the environment under different inlet wind speeds of radiators in the calculation area are as follows. Table 3 lists four typical operating conditions, and Figure 12 shows the temperature distribution for the corresponding operating conditions.
The indoor air temperature distribution cloud map corresponding to the working conditions in the table above is shown below.
The vertical section temperature distribution map shows that when the inlet wind speed of the convective radiator is relatively low, the temperature gradient is larger, and heat is concentrated above the heating area. Therefore, the area of the lower low-temperature area is larger. With the continuous increase in imported wind speed, first, the heat dissipation capacity gradually increases, and the heating capacity of the radiator improves; second, the indoor airflow convection becomes more intense, expanding the area of high-temperature regions and reducing the area of low-temperature regions until the temperature of the entire space is basically uniform. Figure 13 shows the variation in skin temperature on various parts of the human body with the variation in the inlet wind speed of the radiator. The skin temperature of various parts of the human body first increases and then decreases with the increase in inlet wind speed, and there is a significant fluctuation in hand temperature at a speed of 3 m/s. This indicates that an increase in the inlet wind speed of a forced convection radiator can improve the heat dissipation of the radiator, but it can also lead to intensified indoor disturbances, improved heat transfer performance between the air and the human body surface, and a lower temperature of the human body surface, thus reducing the thermal comfort of the human body.
As shown in Figure 14, which concerns the air velocity data points in the vertical direction between the radiator and the human body, it is shown that when the inlet velocity is below 3 m/s, the velocity change at the same position is not significant. Although there are fluctuations in each position, the overall distribution of velocity values is concentrated. However, when reaching a certain speed, there is a significant increase in the velocity in the flow field, which is about 3 m/s. This change will significantly change the air flow state, affect the heat transfer performance of the human body surface, and thus have a significant impact on the surface temperature of the human body. As the inlet wind speed increases, the temperature gradient in the vertical direction changes from a basic linear type to a step type. The range of similar temperatures in the vertical direction continues to expand and finally gradually mixes evenly, forming a stable and uniform temperature zone within a certain space. The temperature step phenomenon in Figure 15 illustrates this phenomenon.

4.2. Heating Effect of Radiators Using Different Air Supply Methods

According to the code regarding thermal comfort (ISO7730) [33], the discomfort of the human body in the local thermal environment can affect overall thermal sensation. The most common cause of local discomfort is the feeling of blowing, and at the same time, both the vertical deviation between the head and the ankle and the asymmetric radiation temperature can be too high, which can also cause local discomfort. On this basis, we studied the local skin temperature distribution of the human body under the conditions of top air supply and side bottom air supply. The calculation results are listed in Table 4.
It can be seen that the local skin temperature difference is mainly reflected in the head and hands. In the top air supply mode of the radiator, the temperature of the head area is higher when the wind speed is low due to the large temperature gradient in the environment. The skin temperature of the head decreases significantly at higher wind speeds (wind speeds greater than 3.5 m/s). At low wind speeds, the skin temperature of the hands in the top air supply mode is hotter than that in the bottom horizontal air supply mode. At high speeds (wind speeds greater than 3.5 m/s), the skin temperature of the hands in the top air supply mode is relatively lower. When using a vertical air supply system at the top of the radiator, when the wind speed is low, due to the uneven mixing of indoor air flow, temperature stratification becomes obvious. The temperature in the area where the head is located is high, and the temperature of the head skin consistently remains at 34.7 ± 0.1 °C. Due to the thermal resistance of clothing, the use of the top air supply mode and the side bottom air supply mode has little effect on local skin temperature indicators of the body (trunk, limbs, and feet). There is not much difference in skin temperature between these two technical solutions, but vertical air supply can cause a large amount of hot air to accumulate in the upper part of the indoor space, resulting in a large indoor temperature gradient. The lower horizontal air supply method can reduce this phenomenon. The different air supply methods have a significant impact on the thermal comfort of the human head and hands, and using the bottom-side air supply is better than using the top-air supply in ensuring thermal comfort.

4.3. Research on Low-Temperature Heating Characteristics of Convective Radiators

In this section, we adopt a comparative analysis method to study heating effects under several operating conditions, including no heating, high-temperature natural convection heating, and low-temperature heating at different wind speeds. Table 5 lists the various operating conditions analyzed. Scenario 1 is the base condition.
Figure 16 shows the temperature distribution in the vertical direction under eight operating conditions. It can be seen that without heating, the average indoor temperature is 16.76 °C. When the water supply temperature is 80 °C, under natural convection conditions, the average indoor temperature can reach 22.04 °C. When using a low-temperature water radiator for heating, the inlet temperature is 40 °C, and the average indoor temperature can reach 18.98~19.58 °C. In terms of the temperature gradient, the vertical temperature gradient of high-temperature natural convection heating is the largest, and a large amount of heat is concentrated above the room being heated. When using lower-temperature heating, the temperature gradient is significantly reduced. As the inlet wind speed of the radiator increases, the vertical temperature gradient is further reduced, ultimately reaching a basically uniform state (a temperature gradient close to 0.2 °C/m). Upon increasing the inlet wind speed of the radiator, the heat dissipation of the radiator continues to increase, but the average indoor temperature does not maintain the same growth trend. It reaches its maximum value at 2 m/s, and when the wind speed continues to increase, it shows a decreasing trend. There are two reasons for this: first, the heat dissipation of the radiator will not increase continuously with the increase in wind speed; second, the indoor air flow intensifies, and the intensity of convective heat transfer between the indoor and wall surfaces strengthens, especially for the partition walls between low-temperature environments, which improve heat transfer capacity and leads to a further increase in heat loss.
As shown in Figure 17, when the water supply temperature of the radiator is 40 °C, as the inlet wind speed increases, the surface temperature of the human body first increases and then decreases. At a speed of 3 m/s, it basically reaches a high value. Continuing to increase the wind speed, as the indoor convection intensity further increases, the surface temperature of the human body will decrease. Among the body parts analyzed, the hands uncovered by clothing and the feet farthest from the position of the heart have larger fluctuations in average skin temperature with outdoor environmental changes. The temperature of the skin on the head and hands without clothing coverage is lower than that without heating when the wind speed is above 3 m/s. This indicates that although forced convection measures can increase the heat dissipation of the radiator, their contribution to indoor human thermal comfort does not always have a positive effect. After the wind speed reaches a certain level, the heat dissipation of the radiator increases, but the skin temperature of the human body decreases, and the thermal comfort effect deteriorates.

4.4. Comparative Study on Low-Temperature Heating Effects of Floor Radiation and Radiators

This article compares the effects of low-temperature heating with convection radiators and floor radiation heating and studies the differences in the thermal physiological effects of the two heating methods on the human body. Heating conditions with indoor average temperatures that are close to each other were compared. The main research conditions are listed in Table 6. According to the references [33], the vertical temperature difference around the human body is one of the factors that affect the thermal comfort of the human body. The surrounding environment affects the human body’s thermal sensation through heat exchange with the human body. The local skin temperature reflects the combined effects of heat exchange between the human body and the environment, as well as temperature regulation mechanisms, and can be used to evaluate the effects of these factors on human thermal sensation. This article uses indoor temperature gradients and local skin temperature of the human body to comprehensively evaluate the heating effects of convective radiator heating and floor radiation heating.
In the above two cases, the indoor reference point temperature is as follows.
Figure 18 and Figure 19 present the temperature distribution of the indoor vertical sections under four comparative operating conditions in the form of temperature cloud maps and statistical data graphs, respectively. Research has shown that there is a significant difference in the indoor temperature gradient between radiator heating and floor radiation heating when the average indoor temperatures are close. The top enrichment effect of hot air in radiator convection heating is significant, with an average vertical temperature gradient of 0.99 °C/m, while there is an average vertical temperature gradient of 0.243 °C/m for floor radiation heating. This indicates that the indoor temperature distribution of floor radiation heating is more uniform, the vertical temperature difference around the human body is smaller, and the thermal comfort of the human body is better.
Figure 20 shows a comparison of skin temperatures of various parts of the human body using convective radiators and floor radiation heating under three conditions of average indoor temperatures of about 17 °C, 19 °C, and 21 °C. It can be seen that when the indoor temperature is below 18 °C and the average indoor temperature is close, the skin temperature of various parts of the human body under floor radiation heating is significantly higher than that under radiator heating. As the indoor temperature further increases, the temperature distribution values of the human skin for the two heating methods are similar. The difference in the skin temperature of human hands is most significant between the two heating modes. As the indoor temperature increases, the skin temperature change induced by radiator heating is greater than that for floor radiation heating. This indicates that when the indoor temperature is lower, the heating effect of the radiator is worse, and the skin temperature of the human body is lower. Overall, the skin temperature induced by floor radiation heating is higher than that for radiator heating. The sensitivity of different parts of the human body to the two technical solutions varies significantly, and the difference in skin temperature between the hands and feet best reflects the difference in the effects of these two technical solutions on human thermal comfort. When the indoor temperature exceeds 19 °C, the difference in skin temperature between the two heating methods is not significant when the average indoor temperatures are similar. The skin temperature of the human feet under floor radiation heating is significantly higher than that under radiator heating. Research has shown that after using forced convection measures to improve the low-temperature thermal performance of radiators, compared with floor radiation heating under similar indoor temperature conditions, when the indoor temperature is higher than 20 °C, the two schemes induce similar skin temperatures on various parts of the human body, and the overall levels of thermal comfort are comparable.

5. Research Conclusions

In this study, we coupled human metabolic factors with heating environmental factors and used a 57-node human thermal physiological model based on the STAR CCM+ platform to study the heating effect of radiators. This article evaluates the effectiveness of forced convection radiator heating from the perspective of the local thermal comfort of the human body and demonstrates the feasibility of this scheme by comparing it with floor radiation heating. It provides a way to objectively calculate and directly quantify the effect of heating equipment on human thermal physiological parameters. The main conclusions are as follows:
  • When the wind speed is less than 3.5 m/s, the increase in wind speed has little effect on the thermal comfort index of the head. The different air supply methods have a significant impact on the thermal comfort of the human head and hands. The use of bottom-side air supply provides better thermal comfort compared to top-side air supply.
  • As the wind speed increases, the surface temperature of the human body first increases and then decreases. At a speed of 3 m/s, it reaches a high value. Upon continuing to increase the wind speed, as the indoor convection intensity further increases, the surface temperature of the human body will decrease, leading to a decrease in thermal comfort.
  • After using forced convection measures to improve the low-temperature thermal performance of radiators, compared with floor radiation heating under similar indoor temperature conditions, when the indoor temperature is higher than 20 °C, the two schemes induce similar skin temperatures on various parts of the human body, and the overall levels of thermal comfort are comparable.
The research results in this article provide technical guidance for the application of convective heat radiators; the widely used plate radiator can be converted into a forced convection radiator by adding turbulence fans, with a recommended wind speed of 2.5 m/s−3 m/s. The heating effect of the piece of equipment using the lower horizontal air outlet scheme is generally better than that of the upper vertical air outlet scheme. Due to the large temperature gradient in the room being heated by the convection-type low-temperature heating radiator, high-temperature air gathers at the top of the room, so the room height should not be too high. Therefore, this type of radiator is suitable for use in residential buildings.

Author Contributions

Conceptualization, Z.L. (Zongjiang Liu).; methodology, L.Z.; software, J.L.; validation, W.X.; investigation, Z.L. (Zongjiang Liu).; resources, Z.L. (Zhong Li).; data curation, W.X.; writing—original draft preparation, Z.L. (Zongjiang Liu); writing—review and editing, Z.L. (Zongjiang Liu).; visualization, J.L.; supervision, W.X.; project administration, L.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the National Key Technology R&D program in the 13th Five Year Plan of Research on Low-Cost Clean Energy Heating and Heat Storage Technology in Villages and Towns (No. 2018YFD1100700) (Subject name: Study on Solar Heating and Heat Storage Technology in Rural Area (2018YFD1100701)).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

Authors Zongjiang Liu, Wei Xu, Zhong Li, and Ji Li were employed by the Institute of Building Environment and Energy, China Academy of Building Research Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. View of climate laboratory.
Figure 1. View of climate laboratory.
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Figure 2. CFD model based on climate laboratory.
Figure 2. CFD model based on climate laboratory.
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Figure 3. CFD simulation boundary conditions.
Figure 3. CFD simulation boundary conditions.
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Figure 4. Human multi-node TCM model created on the STAR-CCM+ platform.
Figure 4. Human multi-node TCM model created on the STAR-CCM+ platform.
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Figure 5. Comparison of head temperatures.
Figure 5. Comparison of head temperatures.
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Figure 6. Comparison of torso temperatures.
Figure 6. Comparison of torso temperatures.
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Figure 7. Comparison of left-upper-arm temperatures.
Figure 7. Comparison of left-upper-arm temperatures.
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Figure 8. Comparison of left-hand temperatures.
Figure 8. Comparison of left-hand temperatures.
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Figure 9. Comparison of left-upper-leg temperatures.
Figure 9. Comparison of left-upper-leg temperatures.
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Figure 10. Comparison of left-foot temperatures.
Figure 10. Comparison of left-foot temperatures.
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Figure 11. Internal details of convective radiator. Main components: 1—upper cover plate with strip seam air supply outlet; 2—water supply manifold on radiator; 3—heat dissipation fins longitudinally installed on the plate; 4—eddy current generator; 5—disturbance fan; 6—return water collection pipe; 7—fresh air fan; 8—lower cover plate with return air outlet; 9—metal radiator convection cover; 10—lower return air inlet of radiator.
Figure 11. Internal details of convective radiator. Main components: 1—upper cover plate with strip seam air supply outlet; 2—water supply manifold on radiator; 3—heat dissipation fins longitudinally installed on the plate; 4—eddy current generator; 5—disturbance fan; 6—return water collection pipe; 7—fresh air fan; 8—lower cover plate with return air outlet; 9—metal radiator convection cover; 10—lower return air inlet of radiator.
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Figure 12. Temperature distribution of indoor air under different inlet wind speeds of radiators.
Figure 12. Temperature distribution of indoor air under different inlet wind speeds of radiators.
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Figure 13. The effect of wind speed on human skin temperature.
Figure 13. The effect of wind speed on human skin temperature.
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Figure 14. Air velocity distribution.
Figure 14. Air velocity distribution.
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Figure 15. Temperature distribution.
Figure 15. Temperature distribution.
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Figure 16. Indoor temperature gradient under different working conditions.
Figure 16. Indoor temperature gradient under different working conditions.
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Figure 17. Comparison of human skin temperatures under different working conditions.
Figure 17. Comparison of human skin temperatures under different working conditions.
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Figure 18. Temperature cloud maps of floor radiation and convection radiator heating.
Figure 18. Temperature cloud maps of floor radiation and convection radiator heating.
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Figure 19. Vertical temperature distribution of floor radiation and radiator heating.
Figure 19. Vertical temperature distribution of floor radiation and radiator heating.
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Figure 20. Distribution of local skin temperature of the human body under different working conditions.
Figure 20. Distribution of local skin temperature of the human body under different working conditions.
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Table 1. The components of the body in the thermophysiological model.
Table 1. The components of the body in the thermophysiological model.
I—HeadII—Upper Right ArmIII—Lower Right ArmIV—Right HandV—Upper Right Leg
VI—Lower Right LegVII—Right FootVIII—Upper Left ArmIX—Lower Left ArmX—Left Hand
XI—Upper Left LegXII—Lower Left LegXIII—Left FootXIV—TorsoXV—Central Blood System
Table 2. Regression equation concerning human skin temperature and environmental temperature.
Table 2. Regression equation concerning human skin temperature and environmental temperature.
HeadUpper ArmForearmHandDorsum
y = 0.06x + 33.35y = 0.09x + 32.69y = 0.11x + 32.02y = 0.17x + 30.53y = 0.20x + 29.65
TorsoAbdomenThighLower legFoot
y = 0.17x + 30.68y = 0.05x + 34.92y = 0.14x + 30.43y = 0.13x + 29.93y = 0.16x + 31.19
Average skin temperature: y = 0.13x + 31.35 (room average temperature: 23 °C, 28 °C, 33 °C, and 38 °C)
Table 3. The four working conditions applied to analyze the indoor temperature distribution.
Table 3. The four working conditions applied to analyze the indoor temperature distribution.
Condition 1The average skin temperature is 30.2 °C, the heat dissipation of the radiator is 647.3 W, and the inlet wind speed of the radiator is 0.2 m/s.
Condition 2The average skin temperature is 30.37 °C, the heat dissipation of the radiator is 741.94 W, and the inlet wind speed of the radiator is 0.8 m/s
Condition 3The average skin temperature is 30.6 °C, and the heat dissipation of the radiator is 964.61 W. The inlet wind speed of the radiator is 2.0 m/s.
Condition 4The average skin temperature is 30.61 °C, and the heat dissipation of the radiator is 1097.74 W. The inlet wind speed of the radiator is 3.0 m/s.
Table 4. Comparison of human local skin temperatures under conditions generated by different air supply modes.
Table 4. Comparison of human local skin temperatures under conditions generated by different air supply modes.
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Table 5. Analysis of low-temperature heating scenarios for convective radiators.
Table 5. Analysis of low-temperature heating scenarios for convective radiators.
ScenariosCondition Characteristics
Scenario 1
(BASE)		(Unheated) Inlet wind speed = 0 m/s, water inlet temperature = 20 °C, radiator flow rate = 0 g/s.
Scenario 2(High water supply temperature, natural convection) Inlet wind speed = 0 m/s, water inlet temperature = 80 °C, radiator flow rate = 7 g/s.
Scenario 3Inlet wind speed = 0.5 m/s, water inlet = 40 °C, radiator flow rate = 7 g/s.
Scenario 4Inlet wind speed = 1 m/s, water inlet = 40 °C, radiator flow rate = 7 g/s.
Scenario 5Inlet wind speed = 2 m/s, water inlet = 40 °C, radiator flow rate = 7 g/s.
Scenario 6Inlet wind speed = 3 m/s, water inlet = 40 °C, radiator flow rate = 7 g/s.
Scenario 7Inlet wind speed = 4 m/s, water inlet = 40 °C, radiator flow rate = 7 g/s.
Scenario 8Inlet wind speed = 6 m/s, water inlet = 40 °C, radiator flow rate = 7 g/s.
Table 6. Convection radiator heating and floor radiation heating conditions.
Table 6. Convection radiator heating and floor radiation heating conditions.
Scenario 1Condition 1 (Low-temperature condition): Clothing thermal resistance = 1.1 clo. Inlet wind speed = 1 m/s, inlet temperature = 40 °C, and radiator flow rate = 7 g/s.
Scenario 2Comparison of condition 1: Floor radiation heating, floor temperature = 25 °C, clothing thermal resistance = 1.1 clo.
Scenario 3Condition 2 (Low-temperature condition): Clothing thermal resistance = 1.1 clo. Inlet wind speed = 3 m/s, inlet temperature = 55 °C, and radiator flow rate = 7 g/s.
Scenario 4Comparison of condition 2: Floor radiation heating, floor temperature = 28 °C, clothing thermal resistance = 1.1 clo.
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Liu, Z.; Xu, W.; Zhang, L.; Li, Z.; Li, J. Research on the Heating Effect of a Convection Radiator Based on a Human Thermophysiological Model. Buildings 2024, 14, 199. https://doi.org/10.3390/buildings14010199

AMA Style

Liu Z, Xu W, Zhang L, Li Z, Li J. Research on the Heating Effect of a Convection Radiator Based on a Human Thermophysiological Model. Buildings. 2024; 14(1):199. https://doi.org/10.3390/buildings14010199

Chicago/Turabian Style

Liu, Zongjiang, Wei Xu, Linhua Zhang, Zhong Li, and Ji Li. 2024. "Research on the Heating Effect of a Convection Radiator Based on a Human Thermophysiological Model" Buildings 14, no. 1: 199. https://doi.org/10.3390/buildings14010199

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