1. Introduction
Nowadays, the three-temperature frost-free refrigerator is popular in China due to its multiple temperature zones, large volume, fast cooling, and automatic defrost. It usually includes a fresh food compartment (FFC), a variable temperature compartment (VTC), and a freezer compartment (FZC) with thermal insulation. Besides, a single evaporator is utilized with the axial/centrifugal fan and separate air ducts to distribute cooled air to all compartments for food conservation. The refrigerator has become the most energy-consuming domestic appliance, so its energy performance has been studied extensively by researchers. It is essentially an enclosed structure with cooling capacity supplied through air circulation, so the refrigeration system, insulation, auto-defrost arrangement, and air circulation/allocation are of great significance to the refrigerator performance.
Many studies were conducted on the refrigeration system and its control strategy for frost-free refrigerators. Yoon et al. [
1] studied the effect of several factors on the energy performance of a dual-evaporator refrigerator, including refrigerant charge, capillary tube, and refrigerant recovery strategy. Results showed that a 7.8% energy saving was achieved by optimizing the refrigerant charge and R-capillary tube, and another 1.8% by improving charge recovery operation. Bjork and Palm [
2,
3] explored refrigerant distribution in a refrigerator under both transient and steady-state conditions, respectively. They found that losses due to charge displacement in the shut-down and start-up processes were 11% in capacity and 9% in efficiency. Visek et al. [
4] studied a sequential dual-evaporator prototype by using phase change material to increase the evaporation temperature during FFC cooling. Results showed that with a condenser fan installed, the FFC-cycle energy consumption reduced by 19.9% and the overall energy consumption decreased by 5.6%. Lu and Ding [
5] adopted the combination control strategy for a parallel two-evaporator refrigerator based on temperature and time-sharing, and reduced compartment temperature fluctuations with much food put into FFC or FZC in a short time or with an extremely high ambient temperature. Zhang et al. [
6] optimized the flow field of a spiral wire-on-tube condenser by adding structural components, so as to reduce the airflow that bypassed the condenser without actually transferring heat. The result showed that the condensation temperature dropped by 0.8 °C and the daily energy consumption by 2.37% of the refrigerator. Zhao et al. [
7] adopted the pump-down operation between FFC and FZC cycles in a parallel dual-evaporator refrigerator, and mitigated the starvation in the FFC evaporator during the initial stage of FFC cycles. Consequently, part of the capacity loss during cycle switches was recovered. Cofré-Toledo et al. [
8] evaluated the integrated evaporator in a refrigerator that utilized two kinds of phase change materials. Results showed that the average temperature of the M-packs in the evaporator increased in general while that in FFC and FZC decreased. Xu and Hrnjak [
9,
10,
11] studied the oil dynamics in the plenum, the discharge pipe, and the separators to obtain a deep understanding of the compressor. Besides, they developed a set of tools to study the behavior of the oil droplet in different compressor plenums. Bansal et al. [
12] explored the heat transfer performance of the ’egg-crate’-type evaporator by varying several parameters. Then, they proposed a geometrically improved evaporator, which had maximum heat transfer capacity in unit weight.
Others paid attention to the effects of thermal insulation on the energy consumption of the refrigerator. Boughton et al. [
13] found that the thermal load through walls and doors accounted for 60% of that in the entire refrigerator. They proposed super-insulations as the vacuum insulation panel (VIP) to be installed to reduce the thermal load. Compared with regular polyurethane or cyclopentane, VIP has a thermal conductivity 80% lower; hence, it can effectively reduce energy consumption. Thiessen and Melo [
14] attached 8-mm thick VIP to the inner side of the compartment steel shell, and applied reverse heat leakage tests to study the effect of VIP coverage area and positions. They suggested that the doors and the rear wall were promising regions to install VIP to reduce energy consumption. Hammond and Evans [
15] embedded VIPs into polyurethane-foamed walls of the refrigerator, and numerically calculated the potential energy savings and payback periods. The average payback was 9.7 years for refrigerators and 4.5 years for freezers. Trias et al. [
16] presented an analytical Lagrange multipliers model to identify possible improvements for the refrigerator to reduce energy consumption or increase the available volume. The VIPs were necessarily embedded into sidewalls and doors to build a highly efficient direct-cooling refrigerator (A++ and A+++) but with a reasonable wall thickness. Sevindir et al. [
17] analytically studied the optimum location of thermal insulation panels with a given cost of the refrigerator. They suggested that if the temperature difference varies with time, the location of vacuum insulation panels should vary accordingly to obtain better heat transfer and energy consumption. Sim and Ha [
18] experimentally analyzed the heat transfer characteristics through the insulating material by using the reverse heat loss method in the refrigerator freezer with VIPs for freezer sidewalls. The overall heat transfer coefficient was derived from minimizing the optimal heat loss function. Afonso and Castro [
19] investigated the magnetic door seals in a domestic refrigerator quantitatively. Results showed that for the tested refrigerator, the deterioration of magnetic seals brought about 505% more air infiltration and 341% higher overall energy consumption. Huelsz et al. [
20] developed a method to assess the thermal load through the door gasket, and found that it contributed to 5.3% of the overall thermal load of the freezer. Kim et al. [
21] presented an approach to evaluate the heat transfer performance near the door gasket for a household refrigerator. The effect of design parameters on the heat loss through the gasket was studied, so as to reduce the energy consumption of the refrigerator.
Frosting and defrosting accounted for a large portion of energy consumption of the frost-free refrigerator. Therefore, efforts were also made to explore the frosting and defrosting process. Li et al. [
22] installed an enthalpy exchanger in the FFC air duct to reduce water vapor entering the evaporator. Results showed that the enthalpy exchanger reduced frost accumulation by 18.8%, and decreased the energy consumption for defrosting. However, that for cooling cycles increased owing to the decrease in the evaporation temperature and air flowrate in the air duct. Zhang et al. [
23] retarded evaporator frosting by dehumidifying the inlet air of the evaporator via a desiccant-coated heat exchanger, and the desiccant was regenerated by refrigerant condensation heat. Theoretical analysis showed the COP of the refrigeration system was within 1.5–2.5 at the ambient temperature of 15–35 °C, strongly dependent on the refrigerator return-air humidity and the freezer return-air temperature. Li et al. [
24] added baffles in the return air duct and provided a better match between frost coverage and defrosting heat dissipation, so as to reduce the defrosting duration by making the defrosting process synchronous over the evaporator. Consequently, the defrosting efficiency was improved by 29.8%. Maldonado et al. [
25] compared three fan control strategies experimentally and proposed the most proper defrost starting time to achieve lower daily energy consumption. No unique fan control strategy assured the most energy-efficient operating mode, which depended strongly on frost accumulation on the evaporator. Liu et al. [
26,
27] proposed two new defrosting methods with outdoor air and phase-change-material thermal storage to improve the defrosting performance. Results showed that the outdoor air or phase-change-material thermal storage serving as defrosting energy could reduce the defrosting power consumption by more than 70%. Melo et al. [
28] evaluated the defrosting performance for three kinds of electric heaters, of which the highest defrost efficiency was 48% while the highest temperature rise was 12.4 °C. They suggested the calrod type to be the best choice when considering both its compatible defrosting performance and low cost. Zhao et al. [
29,
30,
31] found that for the prevailing electric heater defrosting method in frost-free refrigerators, frost-heat mismatch and defrosting warm air intrusion are the root of high defrosting energy and FZC temperature rise in electric heater defrosting cycles. By introducing a dual-heater arrangement to better match the frost coverage and defrost heat transfer and a special fan cover to block warm air intrusion, the FZC temperature rise was reduced by 2.7 °C and the overall energy consumption by 1.2% for the defrost cycle. Knabben et al. [
32] numerically explored the defrost cycle of a 235-W heater, and found that the defrost process took 8.5 min with the heater uniformly covering the evaporator. By contrast, it would take only 3.5 min if the heater is arranged in consistence with frost coverage.
From the above articles, much work has been done on the refrigeration system, the thermal insulation, and the defrost arrangement. However, little attention has been paid to air circulation/allocation through the evaporator, which is actually a key part of the refrigerator since all compartments obtain capacity indirectly from the evaporator-cooled air. A previous work of the authors found that for the three-temperature frost-free refrigerator, the mode of air allocation to compartments significantly affected its energy consumption [
33]. This study, as a supplement, explores air circulation through the evaporator and its effect on the overall performance of the refrigerator. Moreover, better thermal insulation like VIP is proved effective in enhancing refrigerator performance. However, unexpected phenomena often occur during actual use and reduce the energy-saving potential for the refrigerator, necessitating a deeper investigation.
Therefore, this article comparatively examined two refrigerators of the same three-temperature frost-free prototype, one with VIPs and the other without, to investigate the effects of VIPs on both the energy performance and air circulation dynamics of the refrigerator. The whole research was conducted experimentally and numerically. In the experiment part, two refrigerators were tested simultaneously, with both the energy consumption and operation parameter compared to obtain the effects of VIPs on the overall refrigerator performance. In the simulation part, the heat transfer processes through the evaporator were comparatively simulated for the two refrigerators, respectively, so as to study the effect of VIPs on air circulation dynamics. Relations between these two parts were then established, and the thermal load transfer was found from FFC to FZC. The present work will provide guidelines for the manufacturers to design and develop higher grade refrigerators.