Nonuniform Flow Distribution Analysis of Air-Cooled Heat Exchanger

The header and the multiple microchannel tubes connected to the header compose a complicated fluid network with several circuits, and the refrigerant flow into the header and is distributed to the microchannels in parallel by the refrigerant pressure driving in the inlet. So all changed details of geometry, operating conditions and thermophysical properties of the fluids lead to nonuniform refrigerant flow distribution in the microchannels. In the present work, one 6-pass 40-tube microchannel condenser as the research objective was equipped in a window type air conditioner prototype with the cooling capacity 5200W for Middle East T3 climate. A mathematical model based on fluid network theory was developed to predict flow distribution and phase separation in 9 flat tubes and their connecting headers on the second pass of the microchannel condenser. In the assumption of homogeneous flow, the mesh current analysis was employed to solve the mass flow of the loopi+1 by that of the loopi upon two phase pressure drop. The simulated mass flow rate distribution in 9 tubes is parabolic and approaches to uniform distribution when inlet quality comes to the median 0.45 from both directions.

Two-Phase Refrigerant Maldistribution in the Vertical Header and its Effect on the Heat Exchanger Performance as Evaporator in Heat Pump Mode

The vertical header is a usual feature of outdoor heat exchanger of the residential air-conditioning system, typically a multipass microchannel heat exchanger. When operating in heat pump mode, it functions as an evaporator. In such a system, refrigerant maldistribution in the header can deteriorate the performance of the heat exchanger. The objective of study in this paper is the adiabatic upward flow of the refrigerant in the second pass vertical header of microchannel heat exchanger and its effect on distribution. R410A is circulated into the header through the microchannel tubes (5 or 10 tubes) in the bottom pass and exits through tubes (5 or 10 tubes) in the top pass. Three circular headers were explored, each with the microchannel tubes inserted to half depth. The best distribution is found at high flow rate and low quality. The distribution is affected by the flow patterns in the header as well as axial momentum. A distribution correlation, obtained based on the flow rate measurement in each tube, was then incorporated with a microchannel heat exchanger model. The simulation results quantified the capacity reduction of the 2-pass microchannel condenser due to the refrigerant maldistribution.

Numerical Simulation of the Flow Boiling Heat Transfer Performance of R134a in a Microchannel Evaporator With Louvered Fins

The need for more compact and more efficient heat exchangers in the aerospace, automotive, and HVAC&R industries has led to the development of heat exchangers that utilize minichannel or microchannel tubes coupled with louvered fins. Minichannel and microchannel heat exchangers exhibit enhanced heat transfer with a minimal increase in pressure drop over conventional round tube, plain fin heat exchangers often with a significant reduction in the required refrigeration charge and overall heat exchanger size. This paper presents the development and validation of a finite volume, steady-state evaporator model to be used as an aid in heat exchanger design and analysis. The model focuses on evaporator geometries that include minichannel and microchannel tubes with louvered fins and headers. Multiple published correlations provide the user with options for calculating the air-side and refrigerant-side heat transfer and pressure drops within the control volume. Once the model was validated, it was then briefly used to study the effects of maldistribution of refrigerant within the inlet headers on the cooling capacity and refrigerant side pressure drop.

Mocrochannel Evaporators of the Residential Air-Conditioner

The objective of present work was to evaluate the performance of microchannel evaporators of the residential air-conditioner using R-22. Six prototype evaporators were manufactured and tested with psychrometric calorimeter test facilities. The test was conducted in two different ways. Each evaporator consisted of two parallel flow type heat exchangers connected with several return pipes. The heat exchanger had 41 microchannel tubes that had 8 rectangular ports with the hydraulic diameter of 1.3 mm. For the vapor compression system (VCS), the flow area ratio and the number of return pipe had a great effect on the cooling capacity. Type 3 with the flow area ratio of 73/ 58% showed best cooling capacity. The effect of the number of circuit and merging manifold on the cooling capacity was relatively small. For the refrigerant circulation system (RCS), the cooling capacity of the test evaporators was a little bit changed as the mass flow rate and inlet quality increased. The effect of mass flow rate on the cooling capacity was slightly superior to that of inlet quality. The effect of the number of circuit on the cooling capacity was different with the result of the VCS, while the effect of merging manifold was negligible. The cooling capacity proportionally increased as the vertical inclination angle of the evaporator increased due to gravity force.

Feasibility Study on the Prototype Microchannel Evaporator for the Residential Air-Conditioning Application

The present study was aimed to test the feasibility of the prototype microchannel evaporator for the residential air-conditioning application using R-22 refrigerant under wet condition. Eight prototype evaporators were manufactured and tested using a psychrometric calorimeter. Each evaporator consisted of two or three parallel flow heat exchangers connected with return pipes. The parallel flow heat exchanger had 41 parallel microchannel tubes that brazed to the inlet and outlet headers. The tube had 8 rectangular ports with the hydraulic diameter of 1.3mm. The louvered fin had louver angle of 27°, louver pitch of 1.4mm and flow depth of 18.8mm. The cooling capacities of the different test evaporators were severely changed as both mass flow rate and inlet quality were increased due to the flow mal-distribution in the evaporator. The cooling capacity was increased as the vertical inclination angle of the evaporator increased. The condensate under wet condition was also measured. The flow area ratio of the evaporator affected the most seriously among the test parameters on the cooling capacity. Pressure drops on both refrigerant and air sides for the best prototype evaporator were 28.6kPa and 2.5mmAq, respectively.

Mass Flowrate Distribution and Phase Separation of R-22 in Multi-Microchannel Tubes Under Adiabatic Condition

The present study investigated mass flowrate distribution and phase separation of R-22 in multimicrochannel tubes under adiabatic condition. The test section consisted of inlet and outlet headers with the inner diameter of 19.4 mm and 15 parallel multi-microchannel tubes. Each microchannel tube had 8 rectangular ports with hydraulic diameter of 1.32 mm. The key experimental parameters were the orientation of the header (horizontal and vertical), flow direction of refrigerant into the inlet header (in-line, parallel and cross flow) and inlet quality (0.1, 0.2 and 0.3). The effect of inlet quality on the mass flowrate distribution and phase separation in the microchannel tubes was negligible. The effect of the orientation of the header on the flow mass flowrate distribution and phase separation was the largest among the test parameters. Horizontal header showed better mass flowrate distribution and phase separation characteristics than vertical header. Both parallel and cross flow conditions showed better mass flowrate distribution and phase separation than in-line flow condition.