Refrigerant Distribution Research Articles

Flow distribution of two-phase refrigerant in plate heat exchangers

The present study provides experimental observations of air-water and two-phase refrigerant in plate heat exchangers with inlet distributors. A transparent Pyrex plate heat exchanger was used for air-water flow visualization and the Infrared Thermography was used for measuring the surface temperature of a plate heat exchanger with two-phase refrigerant heat transfer inside. The heat transfer performance experiment results and numerical analysis results for air-water flow by Yang et al. (2012) were included for comparison. The air-water mixture flow visualization results show that the effect of Reynolds number on the flow distribution is not high. Flow in the plate with the distributor in the direction 6 provided the most uniform distribution at both high and low Reynolds number conditions. The Infrared thermal images show that the temperature distribution at the cross section of midways along the bulk flow of refrigerant for the plate with distributor in direction 12 are the least in comparing to plates with distributor in other directions. The results of overall heat transfer coefficient test also show that the most uniform refrigerant flow resulted in the highest overall heat transfer performance.

An experimentally validated model for microchannel heat exchanger incorporating lubricant effect

This paper presents the further development of Li and Hrnjak's (2013) microchannel heat exchanger model which includes the thermodynamic and transport properties of refrigerant–oil mixture. Effect of lubricant is accounted in boiling heat transfer, pressure drop and refrigerant distribution. A newly proposed infrared thermography based method is implemented in the model to describe the liquid refrigerant distribution in the inlet header of the microchannel heat exchanger. The new model is validated against experimental results (R134a-PAG 46 oil) at various oil circulation ratios (0.1%–8.3%). Simulation results also indicate that lubricant addition improves refrigerant distribution which is in agreement with experiments and the infrared thermography based method enables the model capture lubricant effect on capacity more comprehensively.

Two-phase refrigerant distribution in an intermediate header of a parallel flow minichannel heat exchanger

Refrigerant R-410A distribution in a two pass evaporator with intermediate header was investigated. The number of tubes was 10 for the first pass and 12 for the second pass. Tests were conducted for the mass flux from 73 to 143 kg m−2 s−1, quality from 0.4 to 0.6. In the intermediate header, two-phase mixture out of the first pass is merged and then re-distributed to the second pass. More liquid is forced downstream as mass flux or quality increases yielding better flow distribution. Effect of insertion device in the inlet header was also investigated. Efforts were made to develop correlations to predict the liquid or gas distribution in a header with limited success. Header pressure drop data are also provided.

Comparison and generalization of R410A and R134a distribution in the microchannel heat exchanger with the vertical header

This article presents the experimental results of R410A and R134a distribution in the vertical header of a reversible outdoor microchannel heat exchanger. Two-phase refrigerant enters into the test header through multiple parallel microchannel tubes in the bottom pass and exits through multiple parallel microchannel tubes in the top pass, representing the flow in the outdoor microchannel heat exchanger of a reversible system under heat pump mode. The difference in fluid properties between R410A and R134a causes the difference in flow patterns and refrigerant distribution results. Non-dimensional analysis shows that the inertia of R410A is higher than that of R134a, with the following results: (1) at low qualities (churn flow), more liquid R410A reaches the top tubes, so R410A distribution is a little better than R134a; (2) at high qualities (semi-annular flow), more bottom tubes are bypassed by the liquid film of R410A, so R410A distribution is worse than R134a, even though more liquid reaches the top tubes for R410A. The distribution results of R410A and R134a are generalized with coefficient of variation, flow regime map, and a distribution function.

Measurement of refrigerant mass distribution within a R290 split air conditioner

R-290 (propane) has been chosen as one of the most potential next generation working fluids of RACs because of its favourable environmental and thermo-physical properties. However, its use is hindered by its flammability and resulting concerns on safety. In addition, the charge mass is limited strictly by the standards which would impact RAC's heating performance. In this paper, refrigerant mass distributions within a R290 split type air conditioner were experimentally investigated at both static and dynamic state, in which the liquid nitrogen method (LNM) was used to determine the refrigerant mass inside the components of the circuit. The distribution of refrigerant that changes with temperatures and compressor speed were also measured and discussed. The results can assist with improving the design of the products (performance improvement, safety measures, etc.) and providing data for further theoretical study and simulation analyses.

Effect of row-crossing header configuration on refrigerant distribution in a two row/four pass parallel flow minichannel heat exchanger

R-410A distribution was experimentally studied for a parallel flow heat exchanger having two row/four pass configuration. The heat exchanger had inlet, intermediate and row-crossing headers. Tests were conducted for the mass flux from 70 to 130kg/m2s with inlet quality 0.2 and exit superheat 5°C. The insert hole size was varied from 2.0mm to 6.0mm. Results showed that the optimum insert hole diameter was 4.0mm. At the hole diameter, thermal degradation was 0.4–5.1%, which increased as mass flux decreased. Flow distribution at the inlet header generally followed the well-known trend. More liquid flowed into downstream channels as mass flux increased. At the intermediate headers, more liquid was supplied into upstream channels. The flow distribution at the front row was not dependent on the insert hole size. On the other hand, the flow distribution at the rear row was heavily dependent on the insert hole size.

An Infrared Thermography Based Method for Quantification of Liquid Refrigerant Distribution in Parallel Flow Microchannel Heat Exchanger

An Infrared Thermography Based Method for Quantification of Liquid Refrigerant Distribution in Parallel Flow Microchannel Heat Exchanger

Two-phase refrigerant distribution in a parallel flow minichannel heat exchanger having lower combining/dividing header

R-410A distribution in a two pass evaporator with lower horizontal combining–dividing header was investigated. Tubes were heated to yield a test section outlet superheat of 5 °C with inlet quality of 0.2. The number of tubes was ten for the inlet pass and 12 or 14 for the outlet pass. For each case, mass flux was varied from 73 to 143 kg/m2 s. In the combining/dividing header, two-phase mixture out of the inlet pass is first merged and then re-distributed to the outlet pass. More liquid is forced downstream as mass flux or quality increases Effect of insertion device in the inlet header was also investigated. Efforts were made to develop correlations to predict the liquid or gas distribution in a header with limited success. Header pressure drop data are also provided.

Quantification of liquid refrigerant distribution in parallel flow microchannel heat exchanger using infrared thermography

This paper presents a method to quantify the distribution of liquid refrigerant mass flow rate in a parallel flow microchannel heat exchanger from the infrared images. Quantification is achieved by building the relationship between the liquid mass flow rate through each microchannel tube and the air-side capacity calculated from the infrared measurement of the wall temperature. After being implemented in a heat exchanger model, the quantification method is validated against experimental data. This method can be used for several types of heat exchangers: evaporators, condensers, gas-coolers, etc., also it can be applied to various heat exchanger designs: different inlet/outlet locations, different flow configurations, etc.

C133 活性炭吸着層内エタノール吸着量分布の可視化・計測(OS4 省エネルギー・コジェネ・ヒートポンプ(2))

It is important for a design of an adsorber to clarify adsorbed refrigerant distribution in adsorbent bed during transient conditions with adsorption or desorption, because the adsorption and desorption process is complex phenomena with heat and mass transfer and strongly effect on the cycle performance of an adsorption refrigerating cycle. Adsorbed refrigerant distributions in an activated carbon particle bed in a metallic rectangular vessel had been quantitatively measured by a neutron radiography method. Ethanol was used as the refrigerant. An umbra method using a checked neutron absorber grid was applied for a quantitative measurement. The measured mass attenuation coefficient of ethanol was 0.386 m^2/kg. It was confirmed from the measurement for an adsorption bed in adsorption equilibrium that the adsorption amount could be quantitatively measured by the method. From the experiments of adsorption and desorption process with heat transfer, it could be clearly visualized that the adsorbed amount in the adsorption process was strongly affected by the position of the heat-transfer surface. In the desorption process, the effect was relatively small, because the heat diffusion in the adsorption bed might be enhanced by regenerated superheated vapor flow.

OPTIMIZATION OF THE RESERVE-TYPE DISTRIBUTOR FOR R410A AIR CONDITIONER

Mal-distribution of refrigerant in a cross-flow type evaporator with parallel paths is very important, which can lead to a loss of heat exchanger capacity to 25%. Distributors are used to balance the two-phase refrigerant distributions in each path. Apart from the structural factors, there are other factors influencing the performance of distributor greatly. In this paper, influences of several nonstructural factors on reservoir distributor are investigated experimentally under varied working conditions. The inlet tube configuration, installation orientation and capillary tube length are also studied. One experiment apparatus is developed to measure the refrigerant mass flow rate and the quality based on R410A air conditioner. It is found that influence caused by inlet tube before the distributor is small. The average STD is only 2.76%. Influence caused by orientation is broad. The average STD is less than 9% on the orientation of 15°. On the orientation of 90°, STDs of different conditions are all more than 40%. For orientation, mass flow rate sensitivity is larger than quality sensitivity. Capillary tubes with different length can be used to adjust distribution. Average STD with sizable capillary length difference is 9.47%. It means that only small mal-distribution can be adjusted by using different-length capillary tubes. Capillary tube length sensitivity increases with the increase of difference between outlet tubes or the decrease of average length of outlet tubes.

Numerical model of two-phase refrigerant flow distribution in a plate evaporator with distributors

The distribution of two-phase refrigerant in the plate heat exchanger has a significant influence on the heat exchanger performance. In order to accurately predict the flow distribution of two-phase refrigerant in a plate evaporator, a numerical model was developed in the present study. In the model, the phase split coefficients for two-phase refrigerant in the distributor were predicted based on the modification of T-junction model, and the empirical factor was correlated with the experimental data; the pressure drops for two-phase refrigerant in the channel, distributor and elbows were predicted by reflecting the structure influences on pressure drop. The model validation shows that, the model agrees well with the experimental data. Based on the proposed model, the influences of distributor angle on the two-phase refrigerant flow distribution in plate heat exchangers were analyzed, and the optimal distributor angle was recommended.

Mitigation of air flow maldistribution in evaporators

Flow maldistribution in evaporators can lead to significant performance and capacity degradation. A significant amount of work has previously been published to overcome this issue. For a fixed air side maldistribution, various methods have been proposed to significantly reduce the effect on performance. Refrigerant compensation has been proposed to reduce the effects of maldistribution in variable air flow systems. However, little work has been found in open literature relative to modifying the refrigerant circuitry to make it less vulnerable to variable air-side maldistribution. The purpose of this paper is to fill this gap in the open literature with a simple and easy to understand case study. The performance of a proposed interleaved circuitry approach, and active refrigerant flow control are compared to standard coil circuitry for different cases of maldistribution. The results show that the interleaved circuitry recovers less of the performance losses than equalization of the exit superheats by active control of refrigerant distribution. However, the implementation cost in an actual system is expected to be much lower and the long term reliability is expected to be better than for an active control approach

Effects of refrigerant charge and structural parameters on the performance of a direct-expansion solar-assisted heat pump system

The direct-expansion solar-assisted heat pump (DX-SAHP) is widely studied as a refrigeration system, which can supply hot water for domestic use during the whole year. The system refrigerant charge and structure parameters are believed to have a great effect on the cycling thermal performance. The refrigerant mass charge including two-phase and single-phase in heat exchangers and pipes is calculated with distributed and lumped parameter approach mathematical models, respectively. Based on the system simulation program, the refrigerant distribution characteristics and system performance under varied structural parameters are obtained. The mathematical calculation results show that the 70%–80% refrigerant charge exists in the condenser and collector; the optimum refrigerant charge, solar collector area, condenser pipe length and condenser internal diameter for the system are 1.65–1.75 kg, 6.0 m2, 70 m and 9 mm, respectively. In the optimum parameters, the better system performance and feasible cost can be achieved.

Effects of fluid properties on two-phase flow and refrigerant distribution in the vertical header of a reversible microchannel heat exchanger – Comparing R245fa, R134a, R410A, and R32

This paper explores the effects of fluid properties on refrigerant distribution in the vertical header of a reversible microchannel heat exchanger under heat pump mode, by comparing the visualization and distribution results of R245fa, R134a, R410A, and R32. Each refrigerant enters into the header through five microchannel tubes in the bottom pass and exits through five microchannel tubes in the top pass. The distribution of R245fa is best, and then R134a, R410A, and R32, because the churn flow region of R245fa in the header is largest, so more tubes have opportunities to receive liquid. For R245fa and R134a, the main reason for maldistribution is because of lacking sufficient momentum to lift liquid to the top. Hence, the distribution may be improved by increasing mass flux in the header at both low and high qualities. For R410A and R32, it is difficult to improve distribution by increasing mass flux at high qualities because it is very easy to form semi-annular flow in the bottom part of the header where the liquid film bypasses the bottom exit tubes and these tubes receive less liquid.

EXPERIMENTAL AND CFD INVESTIGATION OF R410A DISTRIBUTORS FOR AIR CONDITIONER

In order to obtain a higher heat transfer coefficient of refrigerant flow, the diameter of tubes tends to be smaller and smaller, which leads to large pressure drop of the refrigerant flow. Therefore, multiple numbers of parallel refrigerant passages are employed by using distributors. It is very important to distribute the two-phase refrigerant evenly into each tube, otherwise the thermal performance is significantly deteriorated. The performance reduction by flow mal-distribution could be as large as 20–25%. The goal of this paper is to investigate the influence of different configurations to the performance of refrigerant distributors by experiments and computational fluid dynamic code. The effects of mass flow rate and quality of distributor inlet on the characteristics were also quantitatively considered. In this study, an experiment test rig was built to measure the mass flow rate and quality of four circuits after using distributors under different conditions respectively. Refrigerant R410A was used as working fluids. Three classic types (jet, cyclone and reservoir) of distributors with four paths were manufactured and tested under relevant operating conditions. The inlet temperature was 4°C, mass flow rate range was 50–100 kg/h and the quality range was 0.1–0.3. Experimental results show that the maximum deviation of mass flow rate for jet, cyclone and reservoir type is 13.0%, 21.6% and 10.9%, respectively; the maximum deviation of quality was 0.08, 0.10 and 0.05, respectively. In addition, the standard deviation of mass flow rate and quality over four paths were selected to evaluate the performance of different type distributors. The results show that the performance of jet and reservoir are better than cyclone. The flow behavior of two-phase refrigerant such as phase distribution and separation phenomena was studied by Computational Fluid Dynamics (CFD). The flow pattern of inlet for R410A was investigated and used in the present model. The results in the present model show good and reasonable approximation with experimental data which validate the CFD simulation. CFD simulation analysis elucidates the mechanics which shows how the configuration and operation conditions affect the refrigerant distribution.

Characterization of a mini-channel heat exchanger for a heat pump system

In this paper a mini-channel aluminum heat exchanger used in a reversible heat pump is presented. Mini-channel finned heat exchangers are getting more and more interest for refrigeration systems, especially when compactness and low refrigerant charge are desired. Purpose of this paper was to characterize the mini-channel heat exchanger used as evaporator in terms of heat transfer performance and to study the refrigerant distribution in the manifold. The heat exchanger characterization was performed experimentally by means of a test rig built up for this purpose. It is composed of an air-to-air heat pump, air channels for the external and internal air circulation arranged in a closed loop, measurement sensors and an acquisition system. The overall heat transfer capacity was assessed. Moreover, in order to characterize the flow field of the refrigerant in the manifold of the heat exchanger, a numerical investigation of the fluid flow by means of CFD was performed. It was meant to evaluate the goodness of the present design and to identify possible solutions for the future improvement of the manifold design.

Analysis of the performance of the evaporator of automotive air conditioning system

The purpose of this research was to establish a theoretical model for the evaporator of automotive air conditioning system and conducting simulations to evaluate the effect of operation parameters, environmental conditions, and design parameters on the performance of evaporator. An automotive air conditioning system primarily consists of four components: the compressor, the condenser, the refrigerant controller, and the evaporator. The refrigerant flow in the evaporator can be divided into two regions: the evaporating region and the superheat region. The refrigerant in the first region is a two-phase flow, while the refrigerant in the latter region is in the state of superheated vapor. The air flowing through the interior of the evaporator can also be divided into two zones: the unsaturated zone and the saturated zone. Water vapor is condensed in the saturated zone while in the unsaturated zone, no water condenses. Because the refrigerant flow and the airflow are perpendicular to each other, the distribution of refrigerant in the evaporating region and the superheat region does not coincide with the distribution of air in the unsaturated zone and the saturated zone. This study examines the effects of different design parameters, environmental conditions and operating parameters on the cooling capacity and superheat of an air conditioning system. Design parameters include the length of the refrigerant channel, the length of the air channel, and the thickness of the fins. Environmental conditions include the air inlet temperature and absolute humidity. Operation conditions include the refrigerant inlet enthalpy, inlet air flow rate, and refrigerant mass flow rate. Results of simulation demonstrated that fins with 50 micron meters width has the greatest cooling capacity for identical outer dimensions; thicker or thinner fins only decreased cooling capacity. Under different outer dimensions, longer refrigerant tubes and air channels created a greater cooling capacity. However, the increase in cooling capacity becomes less and less if the refrigerant flow was fixed because the heat transfer capability of the gaseous refrigerant was limited. In this study, an increase of 19% in cooling capacity can be reached as the length of refrigerant channels was increased, and the increased length of the air channels can promote the cooling capacity by 22%. Besides, it was found in this study that a decrease in the refrigerant inlet enthalpy, the inlet air flow rate, the air inlet temperature, and the inlet absolute humidity, or an increase in the refrigerant mass flow rate, would extend the superheat region and decrease the refrigerant’s superheat. It was also found that the cooling capacity of air conditioners is extremely sensitive to changes in the refrigerant mass flow rate and the inlet enthalpy, and variations more than 50% were found in the operating ranges examined in this study. However, changes in the inlet temperature, absolute humidity, and inlet air flow rate only resulted in variations between 10% and 20% in the examined ranges of conditions. Finally, a correlation among these variables and the simulated cooling capacity was obtained in this study, enabling the relevant researchers to evaluate automotive air conditioning performance under different environmental conditions and operation parameters more easily.

냉매분배기 분배성능에 미치는 내부 형상인자의 영향

The distribution performance of refrigerant distributors in air conditioner evaporators was examined numerically and experimentally. Internal flow analysis of the distributor by CFD found that the distance from the socket to the cone, the angle of the cone and the base area of the cone were the most important factors affecting refrigerant distribution ability and vortex creation. To enhance distribution performance, two distributors with improved internal structures were designed. To test these new structures, distribution performance was also analyzed by CFD and an empirical experiment was carried out using the water-nitrogen. Experimental results on the distribution fraction of each distributor hole showed a good agreement with the results of the CFD analysis. Thus, the new design of the distributors enhanced distribution performance of the refrigerant distributors.

GAS–LIQUID FLOW SIMULATION IN REFRIGERANT DISTRIBUTOR FOR AIR CONDITIONER

Heat exchangers with small-diameter multi-path tubes have been recently used to improve the efficiency of air conditioners. The difficulty in using tubes with small diameters and multi-paths is the nonuniformity of refrigerant distribution in refrigerant distributors, which results in lower heat-exchange efficiency. Grid methods, such as the volume of fluid method, are now widely used to simulate detailed motions of gas–liquid interfaces. A weak point of grid methods is the numerical diffusion of interfaces that occurs if the scale of interfaces becomes close to the computational grid sizes. We previously developed a particle/grid hybrid method for simulating multi-scale free surfaces. For this study, we modified the hybrid method and applied it to gas–liquid flow simulations in a distributor. The liquid film behaviors in both the distributor and a bend pipe placed in the upstream of the distributor were simulated mainly using the particle method, and gas flows were simulated using the grid method. The predicted liquid film near the outer circumference of the curvature in the bend pipe was thicker than that of near inner circumference of the curvature, which qualitatively agreed with the measurement. The simulated distribution ratio under a steady-flow condition agreed well with the measurement; the predicted distribution ratio was 0.63 and the measured distribution ratio was 0.6.