Microchannel Heat Exchanger Research Articles

Experimental study on heat transfer and flow resistance performance of a microchannel heat exchanger with zigzag flow channels

In order to obtain the heat transfer and resistance characteristics of a microchannel heat exchanger with zigzag flow channels, so as to realize the application of this type of microchannel heat exchanger in nuclear energy development and other fields, the heat transfer and resistance characteristics of the heat exchanger are experimentally studied. By measuring the flow and resistance characteristics of the heat exchanger under various conditions, the influence of various experimental parameters on the heat transfer and resistance performance of the microchannel heat exchanger with zigzag flow channels is analyzed, and the corresponding correlations of heat transfer coefficient and flow resistance factor are established. The research results show that due to the same plate structure, the heat transfer formula expressions of the cold side and the hot side of the microchannel heat exchanger are the same. Therefore, the experimental data obtained in this study can be used to validate the CFD simulation of this type of heat exchanger. Furthermore, the heat transfer coefficient and the flow resistance factor correlations proposed in this study can be used to design this type of heat exchanger.

Experimental study of R452B and R454B as drop-in replacement for R410A in split heat pumps having tube-fin and microchannel heat exchangers

• Drop-in experiments to replace R410A, including R452B and R454B. • Drop-in evaluations in two split heat pumps having different heat exchangers. • Investigations covered two-speed SEER and HSPF rating conditions. • R452B and R454B resulted slightly lower capacities, comparable efficiencies. • R452B and R454B had compressor overheating issues at low temperatures. The objective of this study is to experimentally evaluate two residential split heat pumps using low-GWP refrigerants, i.e., R452B and R454B, and compare the results against R410A. Each heat pump used the same model of two-speed scroll compressor, the same indoor blower and outdoor fan. However, they were distinguished by the indoor and outdoor heat exchanger types: tube-fin heat exchangers (TFHXs) in one case, and microchannel heat exchangers (MCHXs) in the other. The two heat pumps were extensively evaluated following the test conditions for two-speed SEER (cooling seasonal energy efficiency rating)/HSPF (heating seasonal performance factor) ratings. In general, these results show that R452B and R454B are good replacement candidates for R410A for heat pump using tube-fin heat exchangers and microchannel heat exchangers. R452B and R454B led to lower capacities. In the TFHX heat pump, on average, compared to the performance of R410A, the cooling capacity of R452B is 96%; the heating capacity of R452B is 91%; the cooling capacity of R454B is 95%, the heat capacity of R454B is 89%. In the MCHX heat pump, the cooling capacity of R452B is 97%; the heating capacity of R452B is 97%; the cooling capacity of R454B is 98%, the heat capacity of R454B is 97%. With respect to cooling and heating seasonal performances, in the TFHX heat pump, the SEERs of R452B and R454B are 1.5% lower than R410A. HSPFs of the three refrigerants are identical. In the MCHX heat pump, R454B has 7% higher SEER, and R452B has 2% higher SEER than R410A; in heating mode, R454B has the same HSPF as R410A, while R452B is 4% more efficient. The compressor lubricant overheating issue of R452B and R454B, at low ambient heating condition, high stage, should be noticed.

Comparative Numerical Study on Global Heat Transfer Process in Micro-Channel Gas Coolers with Different Structures

A numerical simulation of global heat transfer process in three types of micro-channel heat exchangers was investigated in this paper: spiral double-pipe micro-channel heat exchanger (DPHE), cross-plate micro-channel heat exchanger (CPHE), and shell and tube micro-channel heat exchanger (STHE). The inner tubes of all three heat exchangers have a diameter of 1 mm and are charged with CO2 as refrigerant. A detailed analysis of the heat exchanger's global heat transfer process was carried out, which is entirely different for different structures. The heat transfer characteristics of supercritical CO2 were analyzed by considering the operating pressure, the refrigerant mass flux, the cooling water mass flux, and the heat exchanger refrigerant inlet temperature. The relationship between the CO2 heat transfer coefficient (hCO2) and CO2 bulk temperature (Tb,CO2) was analyzed in detail. The pseudo-critical temperature (Tpc) mainly determines where the peak CO2 heat transfer coefficient occurs. When Tb,CO2 < Tpc , the rise in Tb,CO2 is accompanied by an increase in the heat transfer coefficient, which reaches a maximum when Tb,CO2 is a little bit higher than the pseudo-critical temperature, the heat transfer coefficient curve begins to decline as Tb,CO2 continues to rise. Higher peak heat transfer coefficients can be achieved at higher pressures. Increased refrigerant mass flux always results in larger heat transfer coefficients. The influence of the refrigerant inlet temperature of the heat exchanger in the Tb,CO2 < Tpc region on the heat transfer coefficient is more significant than expected. In this study, different flow patterns on heat transfer due to different structures were compared. The best heat transfer was achieved using a spiral double-pipe micro-channel heat exchanger (DPHE). It consistently reaches the highest heat transfer and the lowest outlet temperature under the same operating conditions‎.

A comprehensive numerical model of a vapour compression refrigeration system equipped with a variable displacement compressor

• A comprehensive numerical model of VCR system is presented. • An oil-free VCR system using linear compressor was measured to validate the model. • Modelling results show a good agreement with measurements with error within ±10%. • The model is able to simulate the compressor and heat transfer in heat exchangers. • VCR system with linear compressor is able to achieve capacity modulation. As one of the most used cooling technologies, vapour compression refrigeration (VCR) has been widely used in domestic cooling, automobile air conditioning, and industrial cooling. Linear compressor can adjust mass flow rate in response to the system requirements to achieve capacity modulation. A numerical model of variable displacement VCR system driven by a linear compressor is presented in this study to study the characteristic of the system, investigate charge impact, and help to optimize VCR system. The model integrates a heat exchanger model, a refrigerant distribution model, and a detailed compressor model. The proposed model is simulated in MATLAB/Simulink. A prototype linear compressor was studied as a case using R1234yf as working fluid in a variable displacement VCR system. Overall, most of the modelling results fall within ± 10 % of the experimental data. The predicted compressor stroke, power consumption, and mass flow rate have Mean Absolute Percentage Errors (MAPEs) of 2.95%, 6.18%, and 7.08%, respectively, while predicted cooling capacity, coefficient of performance, and refrigerant charge have MAPEs of 8.26%, 8.66%, and 6.1%, respectively. The numerical model indicates that a larger compressor stroke tends to have a higher optimal refrigerant charge. The numerical model can be used for future works on low-charge vapour compression refrigeration system design using microchannel heat exchangers and control strategy development for various applications.

Oscillatory valve effect on temperature synchronization in microchannel cooling systems

• Flow maldistribution with oscillation can result in large temperature differences. • Periodic opening and closing of expansion valve can mitigate flow maldistribution. • Only some combinations of valve opening amplitude and frequency can balance flow. • Large amplitude periodic valve opening can balance flow over wider frequency range. • Very small valve opening amplitudes do not synchronize flow for any frequency. Microchannel evaporator cooling systems are widely studied for their ability to remove high heat fluxes. However, flow instabilities during microchannel boiling, like pressure drop oscillation and flow maldistribution, can cause an uneven temperature variation and deteriorate the cooling performance. This study analyzes the effect of using an oscillatory valve opening on the flow distribution and channel temperatures across a two-channel evaporator. Experiments and prediction models indicate that a specific range of valve oscillation amplitude and frequency may cause synchronization resulting in balancing the flow distribution and temperature across the channels, even when they are thermally isolated from each other. Analysis of system performance at different conditions indicates that larger oscillation amplitudes in the valve opening may allow a wider frequency range for temperature synchronization. For a fixed valve opening amplitude, using either significantly small or large frequencies may not be helpful. Moreover, for small amplitudes in valve opening, synchronization may never occur for any frequency. These observations lead us to believe that deliberate oscillations in the valve opening can serve as an active control strategy to mitigate flow maldistribution in parallel channels, suppress temperature oscillation, and improve the overall cooling performance.

Experimental results of a magnetically coupled piezoelectric actuator to relieve microchannel heat exchanger maldistribution

Refrigerant maldistribution is a common issue in microchannel heat exchangers. Refrigerant maldistribution can have significant negative effects on the heat transfer performance and increase the pressure drop, leading to increased superheat temperature at the outlet and increased compressor work, lowering system efficiency. A novel piezoelectric-driven magnetic actuator (PEDMA) was fabricated and inserted into the inlet header of the microchannel evaporator, with the goal of improving the non-uniformity of the flow distribution by regulating the flow in the header to the microchannels. Two prototype PEDMA designs were tested and showed significant impact on the refrigerant flow in the region of actuation. In the first design, the experimental data showed superheat reductions up to 9 °C when the individual channel temperatures were measured. The second design redistributed the refrigerant such that a 1% increase in capacity was calculated for the whole heat exchanger without significantly impacting the heating efficiency. The experimental results demonstrate the successful operation of the PEDMA devices and represent a new method of active flow distribution control which is based on a simple design with minimal additional energy consumption, and which can be inserted into microchannel heat exchanger headers to potentially relieve the refrigerant maldistribution.

Multiobjective geometry optimization of microchannel heat exchanger using real-coded genetic algorithm

• A weight-based, real-coded genetic algorithm optimization was performed. • Air-side Nusselt number, friction factor, and fin efficiency were modeled. • The fin pitch imposed the most significant effect on the volume and the fan power. • The microchannel heat exchanger became more compact at higher target heat duties. • The optimization approach provides optimal solutions at given design dimensions. In this paper, a multiobjective optimization of the structure of a flat-tubed microchannel heat exchanger is performed to reduce its volume and fan power at a specified capacity. Design variables include tube height, tube width, tube length, fin height, and fin pitch. A weight-based, real-coded genetic algorithm is implemented to optimize the design variables within their specified range of dimensions. To further improve the numerical simulations of the microchannel heat exchanger performance, correlations for the air-side Nusselt number, friction factor, and fin efficiency are developed and validated. In the optimization, the Pareto optimal fronts are obtained by varying weights of the two conflicting objectives. A reference microchannel heat exchanger operating at different capacities is optimized. Results show that the volume and fan power of the reference microchannel heat exchanger can be reduced by up to 45% and 51% respectively, depending on the weighting factor selected. The optimization approach of this study provides the optimal solutions at the given domain of geometric parameter dimensions.

A comprehensive study of drop-in alternative mixtures for R134a in a mobile air-conditioning system

• A comprehensive method to evaluate refrigerant alternatives is developed. • No mixture has higher COP, higher Q, lower GWP, and lower flammability than R134a. • Mixing R134a or R227ea with R152a creates a promising alternative to R134a. • The results of mixtures are compared to R134a and guide the design of alternatives. • The method can be extended for searching alternatives for other fluids. R134a will be phased out soon due to its high GWP. Replacing R134a with a low Global Warming Potential (GWP) and non– or slightly flammable fluids in the systems in stock should maintain the same level of system performance. This study focuses on the drop-in alternatives for R134a regarding a Mobile Air-Conditioning system. A simulation model coupled with a microchannel heat exchanger model is developed and validated for calculating the system Coefficient of Performance (COP) and capacity of the Mobile Air-Conditioning system. A comprehensive method is developed to evaluate the alternative candidates combining the performance simulation and the flammability estimated of mixtures. The evaluation includes the system performance, environmental impact, and flammability of alternative mixtures. 15 pairs of binary mixtures from six pure fluids (R134a, R1234yf, R1234ze(E), R32, R227ea, and R152a) with 99 compositions are evaluated as R134a drop-in replacements, and the results are discussed. There is no perfect mixture found that is nonflammable, has low GWP, and has higher system efficiency and capacity than R134a in this study. Using R152a as a drop-in replacement of R134a can increase the system efficiency and capacity, but R152a is flammable. By adding R134a or R227ea to R152a can reduce the flammability of the mixture. R152a/R134a, R152a/R1234yf, and R32/R152a have higher COP and capacity than R134a, but they are not safe or environmental friendly enough. The mixtures for replacing R134a as drop-in alternatives with low Global Warming Potential (less than 150 or 700), nonflammable or slightly flammable are listed and discussed. The method developed in this study can be extended to evaluate alternatives for replacing other working fluids in any thermal application that will be restricted.

Role of beryllium oxide on the thermal efficiency of microchannel heat exchanger with an optimum fin structure

High-efficiency microchannel heat exchangers (MHEs) can remarkably meet the thermal requirements in many areas of technology. Several active and passive techniques can improve the efficiency of heat exchangers; the improvement enables engineering processes to decrease operational costs through waste heat reduction. In the present article, two different passive techniques are applied to improve the heat transfer efficiency of a counter-flow heat exchanger. To do this, four various microchannels with airfoil fins, circular fins, trapezoidal fins, and triangular fins are designed. First, the temperature distribution and effectiveness for each microchannel are calculated and compared with each other. Results of the comparison showed that the optimum structure belongs to the microchannel with trapezoidal fins. Second, BeO ceramic is utilized as the microchannel heat exchanger's material. At a given mass flow rate of 30.5 kg/h, the superior thermal features of BeO lead to more uniform temperature distribution, and also it makes an improvement of 219% in the effectiveness of MHE with trapezoidal fins compared to alumina-made MHE with simple channels.

Experimental analysis of domestic refrigeration system using nanorefrigerant [CeO2+ZnO+R134a

At the present scenario, the action of ensuing the philosophy of reducing energy consumption and saving it for longer period without drop off in performance is increased. On the other hand, global warming and ozone layer depletion become foremost challenges. These concerns are takes place in thermal systems like refrigerator and air conditioning. To resolve the challenges, the nanorefrigerants are used in refrigeration, which has previously got the attention due to its distinctive properties such as thermal conductivity. They also have the potential to improve the heat transfer performance of refrigeration. This project interrogated on the performance of domestic refrigeration system using normal condenser and microchannel condenser with and without nanoparticles. The CeO2 and ZnO nanoparticles in the size of about 20-30 nm and 30-50 nm, respectively, with R134a domestic refrigerant were used. The experimentation carried out using 2 g of CeO2 and ZnO nanoparticles in three different ratios [0.5:1.5, 1:1, and 1.5:0.5] with R134a refrigerant. Hereby, the result conquered that 33.3% increase in the actual COP of domestic refrigeration system using normal condenser with 1:1 ratio of nanoparticles when compared with the refrigeration system using microchannel condenser with and without nanoparticles.

An experimental investigation on the frosting and defrosting process of an outdoor heat exchanger in an air conditioning heat pump system for electric vehicles

The heat pump air conditioning system is a promising solution for energy conservation and ensuring a good driving range can be maintained. Micro-channel heat exchangers are used as outdoor heat exchangers to improve the heat transfer efficiency. A heat pump system introduces frosting problems to the outdoor heat exchanger, especially in a low temperature and high humidity environment. This study conducted an experiment on the frosting and defrosting process of the outdoor heat exchanger of a heat pump air conditioning system for electric vehicles. The surface of the outside heat exchanger had an uneven distribution of the frost layer owing to the variation in the liquid-phase refrigeration occupation. Thermal parameters of the system were obtained. The suction pressure was sensitive to the frosting process, decreasing from (0.15–0.25) to (0.02–0.1) MPa, and it can be used as a frosting monitoring parameter. The frosting and defrosting behavior was also studied at a micrometer scale on the surface of the heat exchanger. The ice bridge phenomenon was observed in the frosting process. Hydrophilic and hydrophobic treatments of the heat exchanger surface were conducted for compassion and it was found that the hydrophilic treatment of the surface of the outdoor heat exchanger of the vehicle heat pump effectively slows frosting. Future experimental research should investigate the effects of superhydrophobic and superhydrophilic surface treatments, in addition to hydrophilic and hydrophobic surface treatments on the performance of the heat exchanger, to improve the performance of the overall heat pump system.

Research on quasi-two-stage compression cycle characteristics of refrigeration system for cold storage

In view of the current test methods and evaluation indicators for the performance of cold storage freezing systems, a cold storage experimental platform with a micro-channel heat exchanger is designed using the inject technology, and the power consumption and cooling performance are tested under different conditions of the internal and external environment temperatures, vapor injecting flux, compressor speed, and vapor inject pattern. The experimental results provide us with a useful source for the optimization or improvement of cold storage freezing systems.

Investigating Flow Instability Mode and Characteristics in a Microchannel Evaporator

Investigating Flow Instability Mode and Characteristics in a Microchannel Evaporator

Experimental Study of Oil Retention in Microchannel Evaporators and Lubricant Effect on Heat Transfer and Distribution Performances with R32/Pve Mixture

Experimental Study of Oil Retention in Microchannel Evaporators and Lubricant Effect on Heat Transfer and Distribution Performances with R32/Pve Mixture

Computational Validation Study of Flow Boiling in a Microchannel Evaporator with Inlet Restrictions

Computational Validation Study of Flow Boiling in a Microchannel Evaporator with Inlet Restrictions

COMPARATIVE EXPERIMENTAL STUDY ON MICROCHANNEL AND FINNED-TUBE EVAPORATORS IN A WINDOW AIR-CONDITIONING SYSTEM

COMPARATIVE EXPERIMENTAL STUDY ON MICROCHANNEL AND FINNED-TUBE EVAPORATORS IN A WINDOW AIR-CONDITIONING SYSTEM

Study on Mathematical Model of Flow Boiling and Flow Instability for Rectangular Expanding Microchannel Heat Exchangers

Study on Mathematical Model of Flow Boiling and Flow Instability for Rectangular Expanding Microchannel Heat Exchangers

Research on Pressure Drop and Heat Transfer Performance of Microchannel Heat Exchangers with Elliptical Concave Cavities

Research on Pressure Drop and Heat Transfer Performance of Microchannel Heat Exchangers with Elliptical Concave Cavities

Analysis of microchannel heat exchanger based on channel geometry

The use of air conditioning in heating ventilating and air-conditioning (HVAC) industry has risen steadily over the last few decades. The goal of the system is to provide a comfortable indoor environment by the process of removal or addition of heat. There are various systems and components which have been in constant use over the years and have evolved with the needs of the user and the environment too. In this research paper attempt have been made to study microchannel heat exchangers in condenser section of package units. The idea is to see how an increase in cross-sectional area through the microchannel increases the area of heat transfer. The effect of grooved cross-section which results in increase of rate of cooling for a fluid flowing through the channel have been studied. R-32 (Freon Refrigerant) is considered as refrigerant along with water for the study using CFD analysis. Two designs were considered for the study considering circular cross section and the grooved cross section considering the channel geometry. From the results of CFD analysis, it can be stated that grooved cross section has performed better in terms of heat transfer and temperature drop as compared to circular cross section.

Geometric Optimization of Complex Heat Exchangers using Constructal Technique

The application of microchannel heat exchangers for cooling is documented in this study. Numerical optimization is carried out with the goal of minimising the peak wall temperature in the microchannel heat sink modelled with solid and half hollow square micro fins. A high-density heat flux q'' from a microelectronic device was applied at the bottom wall of the microchannel heat sink with circular flow channel embedded in a highly conductive rectangular block aluminium substrate. Single phase water was used as coolant to remove the deposited heat at the internal channel walls and in the internal surfaces of half hollow square fins, while cool air stream flows externally over the vertical fins in a forced convection laminar regime to take away the excess heat on the micro square fins. Computational Fluid Dynamics (CFD) code was employed to descretised and solve the governing equations. The results of numerical optimization showed that the combined microchannels with solid square fins has the highest global thermal conductance than the microchannel with half hollow square fins. The limiting case of a single microchannel heat sink without fins agrees with what is in open literature.