**1. Introduction**

The heat pipe (HP) has no moving parts and uncomplicated construction. It is a reliable, and passive heat transfer device—working fluid transport within HP occurs naturally, without additional energy input. Its heat conductance could be higher than any known solid material [1], so it can efficiently transfer heat along significant distances. The separate heat pipe (SHP) is a type of HP, that has a transport or adiabatic section, which separates the evaporator and the condenser section [2]. Because of the existence of the transport section, SHPs are commonly used as heat transferring elements in heat exchangers (HEXs). The type of HEX in which heat pipes are utilized is called a heat pipe heat exchanger (HPHE). Recently, HPHEs have gained popularity in heat recovery applications, e.g., in air conditioning, technological processes, etc. [3,4]. Much attention is focused on testing the capability of HPHEs as recuperators in residential ventilation and air conditioning systems [5]. HPHE intended for heat recuperation in the area of air conditioning, made from a finned bundle of horizontal heat pipes, was tested in [6]. Heat pipes were filled with R-11 refrigerant, and it was found that the peak effectiveness of HPHE is approximately 0.5 (for a range of volumetric flow of air: 1200–3000 m3/h). Baghban and Majideian [7] investigated HPHE as a recuperator for surgery rooms in hospitals. They chose methanol as the SHP's working fluid and tested it for volumetric flow approx. 400 m3/h. The very small effectiveness which they obtained (ε = 0.16) was a result of the utilization of bare HP without any fins. Wu et al. [8] proved experimentally that HPHE can be used effectively as a dehumidifier in air conditioning systems. Their

**Citation:** Górecki, G.; Ł ˛ecki, M.; Gutkowski, A.N.; Andrzejewski, D.; Warwas, B.; Kowalczyk, M.; Romaniak, A. Experimental and Numerical Study of Heat Pipe Heat Exchanger with Individually Finned Heat Pipes. *Energies* **2021**, *14*, 5317. https://doi.org/10.3390/en14175317

Academic Editor: Artur Bartosik

Received: 9 July 2021 Accepted: 21 August 2021 Published: 27 August 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

HPHE consisted of continuously finned copper SHPs with R-22 used as working fluid. Longo et al. [9] made an experimental and theoretical analysis of HPHE consisting of finned HPs with internal micro-fins. For the volumetric airflow range, 400–1000 m3/h HPHE effectiveness peaked at approximately 0.55. Working fluids, in this case, were modern refrigerants: R-134a, R-1234yf, and R-1234ze. Small, continuously finned HPHE was tested in [10] for air volumetric flows close to 40 m3/h (water as working fluid). The highest, recorded effectiveness was ε = 0.6. Rajski et al. [11] performed a theoretical analysis of wickless HP-based HPHE, which worked as an indirect evaporative cooler. For the considered air flows up to 450 m3/h, high coefficients of performance of evaporative cooler were achieved. Yau and Ahmadzadehtalatapeh [12] conducted an interesting experiment on the effects of working fluid charge ratio on HPHE effectiveness. They have shown that optimal effectiveness is attained for a filling ratio exceeding slightly the amount to saturate the HP wick. The above-mentioned HPHEs are based on HPs with wicking structure (operate horizontally as well as vertically) or wickless HP (operate at an inclination that ensures a gravity-assisted return of the condensed working fluid). Although, there are many promising, novel HPHE types, which are made from other, more unusual types of HPs, such as pulsating or micro HPs. Their usage in heat exchangers was presented and analyzed by Vasiliev [13]. A flat micro-fins HP array was investigated as air to the air heat exchanger by Yang et al. [14]. Fresh air flow was kept at 1000 m3/h and outflow air at 1500 m3/h. Maximum recuperation efficiency was found to be 0.83. Recently, Yang et al. [15] have tested the performance of the pulsating heat pipe heat exchanger using deionized water and HFE-7000 as working fluids. For the airflow range, up to 300 m3/h, the maximum effectiveness obtained was approx. 50%. Even for HPs meant to be working as pulsating HPs, the chosen diameter was too large to induce the oscillating motion of fluid, and each of HP pass worked as an individual wickless HP. A few papers were focused strictly on modeling of HPHEs. Brough et al. [16] successfully used TRNSYS software to simulate HPHE response to transient input conditions. The same program (TRNSYS) was applied for prediction of yearly energy recovery and dehumidification intensification resulting from HPHE installation in an air conditioning system in tropics conditions [17]. Yu et al. [18] developed an optimization procedure for segmented separate type HPHE. Righetti et al. [19] compared their own computational model with experimental data for six-rows HPHE. Excellent agreement was obtained for heat transfer rate and pressure drop. It convinced the authors of the present work that the most accurate results could be produced by our own, customized numerical model of HPHE. From the above literature review one can draw the conclusion that examined HPHEs were mainly based on the wick-assisted and wickless HPs. Air heat transfer resistance was reduced mainly by the application of aluminum continuous fins (plate-fin tube assembly). In most of the available research, volumetric flows above 1000 m3/h were considered. The present theoretical study is the only one where HPs are individually finned, based on the literature review carried out by the authors. Theoretical analyses were conducted for a smaller air flow range (370–530 m3/h) than usually stated in the similar experimental works. It makes the present study important in the view of broadening the knowledge about the design of air-to-air HPHEs.
