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Background

Microchannel-tube, multilouver-fin heat exchangers are fast replacing conventional round-tube, plate-fin condensers, particularly in automotive air-conditioning applications. The larger air-side heat transfer coefficients due to the louvers and the larger surface area per unit volume of these heat exchangers are believed to lead to more compact geometries. The basic geometry of a microchannel condenser is shown in Figure 1.

Motivation

This work was driven by the need to improve the efficiency of residential air-conditioning condensers. An improvement in efficiency can lead to more compact geometry, resulting in savings in material cost. Compact geometries can also result in the reduction of required refrigerant charge. Besides savings in cost, reduction in refrigerant charge also leads to environmental benefits in the form of reduced ozone depletion and global warming. Keeping these environmental benefits in mind, refrigerant R-410A, which has zero ozone depletion and global warming potential, was used for this study.

Past Work / Need For Research

Figure 2 Cause of air flow mal-distribution

A typical condenser unit of an air conditioning system is shown in Figure 2. The condenser coils are vertically placed along the four sidewalls of the condenser unit. Air flow over the condenser coils is generated by an induced draft fan placed at the top of the unit. This configuration results in a non-uniform air flow through the condenser coils. Hence a uniform air flow distribution, which is assumed in most theoretical studies, is an idealization. A study of the condenser performance for mal-distributed air flow conditions is required.

Specific Objectives

· Modify the condenser geometry for optimal performance under these mal-distributed air flow conditions

Approach

The condenser performance was numerically analyzed by dividing the condenser into small segments along the length. The heat transfer calculations in each segment were performed using the Effectiveness-NTU method. The refrigerant properties for each segment were calculated based on the conditions and the center of that segment. As the conditions at the center of a segment were not initially known, the calculations were done iteratively with a stop criterion of less than 0.1% variation in the calculated heat duty in successive iterations.

Sample Findings

Figure 4 Optimization for Uniform Air Flow

For a uniform air flow, various condenser geometric parameters were optimized to provide the design heat duty of 14.5 kW with minimum condenser mass. The variation of condenser mass, height and length along this optimization procedure is shown in above.

Figure 5 Effect of air flow mal-distribution

The inlet air flow to the condenser was then mal-distributed linearly up to a 50% mal-distribution. The effect of such a mal-distribution is shown in figure 5. There was found to be a maximum increase of 7% in the required condenser mass due to these mal-distributions.

Figure 6 Optimum design for mal-distributed air flow

The present work highlights the need for condenser design to include the effect of mal-distributed air flow conditions. It was shown that air flow mal-distribution significantly alters condenser performance, so as to justify the use of modified fin distributions to counter this air flow mal-distribution.

The design procedure presented in the current work can be used as a tool for efficient microchannel condenser design. A set of design constraints were used in the current work. These constraints could be modified to suit the specific application. Also the ease of manufacture and first cost of the optimum geometry needs to be considered, while designing for a real-life application.