Parallel Flow Evaporator Research Articles

Thermodynamic performance study on the novel efficient flow pattern global construction evaporator

Based on the fluid-efficient flow pattern global construction heat transfer enhancement mechanism, a novel evaporator, called the efficient flow pattern global construction evaporator (EFGE), is presented to improve the in-tube evaporation heat transfer at low quality. The numerical analysis and experimental study of EFGE's thermodynamic performance are performed. Results show that the temperature distribution characteristic of the fluid-efficient heat transfer flow pattern construction is better than that of the initial fluid. The evaporation heat transfer coefficient (HTC) of EFGE is 0.34–1.04 times that of a common parallel flow evaporator (PFE), and the pressure drop of EFGE is only 80%–116 % of that of a common PFE at the quality of 0.9. The theoretical nonuniformity of the evaporation HTC between low- and high-quality flow is approximately 12%–67 %, which is 55%–72 % of the pressure drop. The numerical analysis results agree with the finding that EFGE has better thermodynamic performance than PFE in terms of friction power reduction and minimum entropy generation number.

Experimental study and analytical modeling of thermosyphon loop for cooling data center racks

This paper presents experimental study and analytical model for a passive two-phase thermosyphon loop designed to remove directly the hot air flow inside a data center. The cooling loop with separated lines working without electricity is tested under different conditions. A mini-channel parallel-flow evaporator and a finned-tubes condenser are used to transfer heat from the data center to outside environment. Experimental tests are conducted with different n-pentane fill ratios. According to these tests, the cooling performance is showed and the refrigerant distribution is specified. The proposed steady-state model is based on the combination of the thermal and hydraulic models of the two-phase flow inside the loop. It well predicts the experimental results such as the mass flow rate and working fluid pressure drops. As investigated by the model, the most parameters influencing the cooling capacity are: the working fluid, the condenser tubes number and the outside temperature.

Influence of fin thickness on heat transfer and flow performance of a parallel flow evaporator

This paper studies numerically the influence of the louver?s fin thickness on heat transfer and flow performance of a parallel flow evaporator, a comprehensive evaluation and analysis of the five structures at different Reynolds numbers are systematically carried out. Comparison of the numerical results with the experimental data shows good agreement with maximal errors of 12.16% and 5.29% for the heat transfer factor and the resistance factor, respectively. The results show that the heat transfer coefficient and the pressure drop increase with the increase of the thickness of the louver fins when the Reynolds number is a constant. The analysis of the comprehensive evaluation factor shows that the A-type fin is the best, and it can effectively strengthen the heat exchange on the air side and improve the heat transfer capacity of the system. The research results can provide reference for the structural optimization of the louver fins.

Experimental and numerical investigation on a CO2 loop thermosyphon for free cooling of data centers

Free cooling based on loop thermosyphon is an ideal method for energy-saving of data centers. In order to promote its future application, it is necessary to investigate the performance of utilizing environment friendly working fluids. In this paper, an experiment based on enthalpy difference method and thermal imaging is conducted on a CO2 loop thermosyphon with microchannel parallel-flow evaporator and condenser. The results show that the effect of filling ratio on CO2 loop thermosyphon is more significant and the optimal filling ratio is higher, compared with R22 loop thermosyphon. Thermal image of the evaporator shows that superheated region shrinks and then disappears with the increase of filling ratio. Moreover, to analyze the relationship between the phenomenon and the internal flow state, a distributed-parameter model is built and validated. The simulation results show that 150% is the optimal filling ratio for no superheating or subcooling occurs along the loop. The effects of height difference and pipe length are more significant for low filling ratio. Superheating is easier to occur at the evaporator outlet when height difference is smaller or pipe length is larger, and subcooling is easier to occur at the condenser outlet when height difference or pipe length is smaller.

An experimental study on refrigerant distribution in a two row/four pass parallel flow minichannel heat exchanger

R-410A distribution was experimentally studied for a parallel flow evaporator having two row/four pass configuration. The evaporator has inlet, intermediate and row-crossing headers. Tests were conducted for the mass flux from 70 to 130 kg/m2s with the quality at the inlet of 0.2 and exit superheat 5 °C. Significant heat transfer degradation (13–40 %) was realized for the two row/four pass configuration due to flow mal-distribution. Of the three insert hole sizes, 4.0 mm hole yielded the least heat transfer degradation followed by 6.0 and 2.0 mm holes. At the inlet header, more liquid flowed into upstream channels. At the intermediate headers, more liquid was supplied into downstream channels. Similar flow distribution was obtained before and after the row crossing header. Header pressure drops were obtained by subtracting the flat tube pressure drops and other minor pressure drops from measured pressure drops.

Quick and empirical correlations for refrigerant pressure drop in mobile air conditioning system evaporators

In heat exchanger design and simulation, a precise refrigerant pressure drop prediction is difficult because of limited correlation's accuracy. In the present paper, several quick and accurate empirical correlations for refrigerant pressure drop of the typical evaporators (laminated plate evaporator and minichannel parallel flow (MCPF) evaporator) used in mobile air conditioning (MAC) system were proposed. A factor which was related with the refrigerant mass flow rate, thermodynamic properties and refrigerant status was introduced. The types of evaporator and refrigerant were considered in the correlations which were available under typical MAC evaporator application conditions. The analysis results showed the refrigerant pressure drops were implied in a linear relationship with the proposed factor. The predicted data agreed with the experimental data very well. For laminated palate evaporators, the average deviations were 1.8% and 0.7% for R134a and R1234yf, respectively. For MCPF evaporator, the average deviations were 4.5% and 0.6% for R134a and R1234yf, respectively.

설비공학 분야의 최근 연구 동향 : 2013년 학회지 논문에 대한 종합적 고찰

This article reviews the papers published in the Korean Journal of Air-Conditioning and Refrigeration Engineering during 2013. It is intended to understand the status of current research in the areas of heating, cooling, ventilation, sanitation, and indoor environments of buildings and plant facilities. Conclusions are as follows. (1) The research works on the thermal and fluid engineering have been reviewed as groups of fluid machinery, pipes and relative parts including orifices, dampers and ducts, fuel cells and power plants, cooling and air-conditioning, heat and mass transfer, two phase flow, and the flow around buildings and structures. Research issues dealing with home appliances, flows around buildings, nuclear power plant, and manufacturing processes are newly added in thermal and fluid engineering research area. (2) Research works on heat transfer area have been reviewed in the categories of heat transfer characteristics, pool boiling and condensing heat transfer and industrial heat exchangers. Researches on heat transfer characteristics included the results for general analytical model for desiccant wheels, the effects of water absorption on the thermal conductivity of insulation materials, thermal properties of Octadecane/xGnP shape-stabilized phase change materials and CO2 and CO2-Hydrate mixture, effect of ground source heat pump system, the heat flux meter location for the performance test of a refrigerator vacuum insulation panel, a parallel flow evaporator for a heat pump dryer, the condensation risk assessment of vacuum multi-layer glass and triple glass, optimization of a forced convection type PCM refrigeration module, surface temperature sensor using fluorescent nanoporous thin film. In the area of pool boiling and condensing heat transfer, researches on ammonia inside horizontal smooth small tube, R1234yf on various enhanced surfaces, HFC32/HFC152a on a plain surface, spray cooling up to critical heat flux on a low-fin enhanced surface were actively carried out. In the area of industrial heat exchangers, researches on a fin tube type adsorber, the mass-transfer kinetics of a fin-tube-type adsorption bed, fin-and-tube heat exchangers having sine wave fins and oval tubes, louvered fin heat exchanger were performed. (3) In the field of refrigeration, studies are categorized into three groups namely refrigeration cycle, refrigerant and modeling and control. In the category of refrigeration cycle, studies were focused on the enhancement or optimization of experimental or commercial systems including a R410a VRF(Various Refrigerant Flow) heat pump, a R134a 2-stage screw heat pump and a R134a double-heat source automotive air-conditioner system. In the category of refrigerant, studies were carried out for the application of alternative refrigerants or refrigeration technologies including CO2 water heaters, a R1234yf automotive air-conditioner, a R436b water cooler and a thermoelectric refrigerator. In the category of modeling and control, Korean Journal of Air-Conditioning and Refrigeration Engineering Vol. 26, No. 12 (2014), pp.605-619 http://dx.doi.org/10.6110/KJACR.2014.26.12.605

Cooling characteristics of ground source heat pump with heat exchange methods

The objective of this study is to investigate the influence of the cooling performance for a water-to-water ground source heat pump (GSHP) by using the counter flow and parallel flow methods. The GSHP uses R-410A as a refrigerant, and its main components are a scroll compressor, plate heat exchangers as a condenser, an evaporator, a thermostatic expansion valve, a receiver, and an inverter. Based on our modeling results, the heat transfer rate of the counter flow evaporator is higher than that of the parallel flow evaporator for a heat exchanger length greater than 0.42 m. The evaporator length of the GSHP used in this study was set to over 0.5 m. The performance of the water-to-water GSHP was measured by varying the compressor speed and source-side entering water temperature (EWT). The cooling capacity of the GSHP increased with increased compressor RPMs and source side EWT. Also, using the counter flow method, compared to the parallel flow method, improves the COP by approximately 5.9% for an ISO 13256-2 rated condition.

The net power output of ideal supercritical organic Rankine cycle with different flow arrangement evaporators

The theoretical models of net power output for ideal supercritical ORC (organic Rankine cycle) with the evaporator of counter flow, parallel flow, and cross flow are, respectively, proposed. The effects of the ratio of heat capacity rates of heat source and working fluid, the number of heat transfer unit, and the ratio of the cycle high and low temperatures on the net power output of ideal supercritical ORC are addressed. The numerical simulation results of ideal supercritical ORC elucidate that the larger rate difference between the heat capacity of working fluids and heat source will help to improve the net power output. The net power output will be kept constant when the number of heat transfer unit reaches a certain value. In addition, supercritical ORC with counter flow evaporator exhibits the largest net power output while one with parallel flow evaporator does the lowest.

대향류와 평행류형 판형 증발기에서 운전방식에 따른 성능특성 분석

The analysis of performance characteristics was carried out in the plate type evaporator with counter and parallel flow. To investigate performance of evaporator with water inlet temperature and refrigerant mass flow rate were changed. As a result, when the inlet temperature of water is <TEX>$8^{\circ}C$</TEX>, capacity of parallel flow evaporator higher than counter flow is 0.35%. But as the inlet temperature of water rises from <TEX>$8^{\circ}C$</TEX> to <TEX>$16^{\circ}C$</TEX>, capacity of counter flow type evaporator higher than parallel flow type is 0.12%, 0.27%, 1.1%, 1.6%, respectively. The findings showed that counter flow type evaporator has a larger capacity than those that were parallel flow type evaporator. As the refrigerant mass flow rate rises, capacity and pressure drop increases in the counter and parallel flow type evaporator.

Process Optimization for the Side-Plate of the Core Part in Parallel Flow Evaporator

The forming scheme of a side-plate of the core part in parallel flow evaporator was studied. Two forming schemes were compared and the explicit dynamic software DYNAFORM was applied to make simulations about the forming processes. According to comparisons between two forming schemes, the optimized scheme and process parameters were determined. Multi-station dies were designed based on the optimized process. Successful experiments with the dies showed that numerical simulation provided an efficient way for the optimization of the forming scheme in the side-plate production.

Experimental investigation of frost formation on a parallel flow evaporator

This paper experimentally studied the frosting process of a folded–louvered-fin, parallel flow microchannel evaporator in a heat pump central air-conditioning system under three conditions, in which three open states of two capillaries were adopted. Surface temperature distribution on evaporator was measured by 16 thermocouples buried on the leeward side. Mesoscale frost formation processes on its front view surface for three different test conditions were observed using a Charge Coupled Device (CCD) camera. The results showed that the surface temperature distribution on the parallel flow evaporator was uneven and initial frost formation generally started on some partial surface areas of the louvered fins whose surface temperature was lowest after the heat pump system running 6 min later under conditions B and C, while the evaporator began to frost after 8 min under condition A. The non-uniform surface temperature distribution caused by the unequal distribution of the refrigerant flux in the flat tubes’ microchannels resulted in uneven distribution of frost. The ice crystals distribution and frost thickness in frost period could be obtained by the digital image processing method in which the initial pictures were converted into binary image. The results indicated that in a thin layer near fins’ surface, ice crystals had relatively high occupancy rate in the frost growing period and full growth period; the occupancy rate of ice crystals decreased almost linearly with the increasing of the frost thickness (frost height), where the decreasing rate in the frost full grown period was less than that in the frost growing period (in 0.1 mm frost thickness condition, the occupancy rate of ice crystals decreased to 58% in the frost growing period, while in the frost full grown period occupancy rate of ice crystals decreased to 90%; and in 0.25 mm frost thickness, they were 0%, 45% respectively). Furthermore, it can be found that the frost thickness increased as the time increasing and then finally reached maximum values of 0.3 mm, 0.35 mm, and 0.32 mm respectively at three conditions.

Experimental Study of Gas-Liquid Two-Phase Flow Even Distribution in Parallel-Flow Evaporator

An efficient parallel-flow evaporator has been applied to cryogenic equipment in recent years. The experiment studied the four different construction forms of parallel-flow evaporator in different conditions, the distribution of liquid flux in tubes, and observed the flow patterns. The following results are obtained: the uniformity of the liquid flux was improved a little and sometimes worsened through increasing the inner diameter to regulate the liquid distribution in the branch pipe; the inlet tube should be arranged in the middle of the header tube rather than sides of it, which is more beneficial to the even distribution of the liquid flux in the branch pipe.

알루미늄 평행류 열교환기에서 냉매패스와 분배량 변화의 영향

본 연구에서는 천정 설치형 공조기 적용을 위해 연구가 활발히 진행되고 있는 알루미늄 평행류 증발기 해석 프로그램을 개발하여 190mm*650mm*25mm(W*H*D) 크기의 열교환기를 해석하였다. R410A냉매의 분배비가 일정하고 상하로 유동하는 2, 3 패스의 경우와 3 패스이고 분배비율(1:1:1, 1:2:2)을 달리할 때, 열교환 성능을 예측하였다. 계산 결과, 2 패스가 3 패스보다 국소 전열량이 30% 정도 높았지만, 건도에 따른 열전달계수는 25% 낮았으며, 3 패스의 경우, 분배비 1:1:1 보다 1:2:2가 냉매 압력손실이 높게 나타났다. 본 해석에서는 패스당 냉매가 균일하게 분포하는 것으로 가정하였으므로, 패스별로 불균일하게 유동되는 실제의 경우보다 열교환성능을 과대 예측하는 것으로 판단된다. In this study, an analysis code was created for a 190*650*25-mm (W*H*D) parallel-flow evaporator, and research was done on how to increase the heat transfer rate of aluminum PF heat exchanger for application in IDU. After varying the R410A refrigerant up-down flow to two and three passes and the distribution ratio to 1:1:1 and 1:2:2, it was determined that the two-pass flow has a 30% higher partial heat transfer rate and a 25% lower heat transfer coefficient compared to the three-pass flow. As for the distribution ratios of the three-pass flow, 1:1:1 was found to have a lower refrigerant pressure loss than 1:2:2 distribution. It was assumed, though, that the refrigerant distribution had a uniform flow and that its value was thus overestimated in the actual case of maldistribution in each pass.

Rehabilitation of a long-tube parallel flow evaporator Al-Ghubrah power plant and desalination station

The No. 1 desalination plant of the Al-Ghubrah power and desalination plant, Sultanate of Oman, is a multi-stage flash long-tube parallel flow type with 18 stages of heat recovery and two stages of heat reject in five vessels. The plant was commissioned in the year 1976. It had experienced severe corrosion in the vapour space, and more in the high temperature stages. The support plate, inter-stage walls, and top roof plate were badly corroded. Tube bundles began to sag and some collapsed in vessel No. 1. Tube leaks were encountered frequently. It was analyzed that corrosion was due to build-up of non-condensable gases in these stages. The plant venting system was designed to have series venting (cascaded) from stage No. 1 to 20. The plant was rehabilitated in 1990. The paper describes the study of corrosion, modifications of the venting system and preventive measures being carried out to prevent the accumulation of non-condensable gases. After modification and tube bundle replacement in vessel No. 1, no significant corrosion has been observed in this vessel even after 5 years of operation.