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The Research on Gas-Liquid Two-Phase Flow Characteristics in Parallel-Flow Heat Exchanger and Process of Frost

Author: WuXiaoBo
Tutor: LiZuoNing
School: Chongqing University
Course: Power Engineering and Engineering Thermophysics
Keywords: Parallel Flow Evaporator Gas-liquid Distribution Resistance Loss Frost Excluded Condensed Water
CLC: TK172
Type: Master's thesis
Year: 2009
Downloads: 293
Quote: 1
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Abstract

Parallel-flow heat exchangers consist of small hydraulic diameter multi-channel flat tubes on the refrigerant-side, and louver fins on the air-side, so air side and refrigerant side heat transfer have been enhanced, been considered one kind of efficient, compact heat exchangers. Parallel-flow heat exchanger has been widely used in automotive air-conditioning system as the condenser, in recent years has been applied to the evaporator, home air conditioners, refrigerators, and many other areas, to replace the conventional finned-round-tube heat exchanger. Gas-liquid (especially liquid) two-phase distribution in each flat tube of the parallel-flow evaporator has a greater impact on the thermal performance, if unevenly distribution in flat tubes, the thermal performance is significantly deteriorated; At low temperature environment parallel-flow evaporator easy to frost, but frost has a great impact on normal operation of the evaporator. With the increasing of the frost thickness, gradually blocked evaporator, so air flow through the evaporator is getting smaller and smaller, and ultimately completely blocked the evaporator. At the same time, the thermal conductivity resistance of frost layer is gradually increasing, seriously affecting the heat transfer; the condensate water of parallel-flow evaporator whether or not export has greater impact on its performance, if the condensed water do not discharged, performance will worsen.This article experiment studied the liquid flux distribution in branch pipe of six different construction forms of parallel-flow evaporator in different gas–liquid flux, which showed that: the flow pattern is mainly annular flow, with the mixture of wavy flow at the bottom of the header tube. The following results are obtained through the experiment: 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 header tube rather than sides of it, which is more beneficial to the evenly distribution of the liquid flux in the branch pipe, applicable to upward flow and downward flow.Through comparison of the experimental results and calculated values , found that: the model is based on homogeneous phase and large-diameter branch pipes, but experiment results is annular flow, that is liquid phase mainly concentrated on the wall of the tube, gas phase concentrated on center of the tube, as well as the resistance loss difference between small-diameter pipe and large diameter pipe, so resulting in friction resistance loss, local resistance loss larger difference between the actual values and the model calculated values. The model is very sensitive to increasing the inner diameter of the tube; however, the actual change is not as great.The study found: there are a variety of air parameters great influence on the frosting rate of the general fin-round-tube evaporator, the greater of air relative humidity, wind speed the more serious the evaporator frosting, the range of frosting temperature is 0℃to peak of the curve. At the same time, also found that heat transfer there is a slight increase in the early frosting of the heat evaporator, but in the late a sharp drop for evaporator frosting. Different regions should be based on different air parameters constitute of different defrost cycle.When parallel-flow evaporator frost layer accumulated, more obvious decline about overall heat transfer coefficient and air-side heat transfer coefficient for big fins depth and height, and from heat transfer and mass transfer analogy can know that: if the overall heat transfer coefficient and air-side heat transfer coefficient slower decline so mass transfer is slow that is slower frosting, so lower the frost growth rate, the rate of pressure drop increase will also reduce, parallel flow evaporators should have proper fin depth and pitch to ensure that the pressure drop, the total heat transfer coefficient, fin efficiency will not change too fast after the frost.Parallel flow evaporator can be placed inclined, evaporator fin can be used hydrophilic (wetting) surface treatment, and to accelerate the condensate water vent out; can also be opened water-diversion channel in the fin. In the structure of parallel flow evaporator can be opened water-diversion channel in the flat tube to excluded water.

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CLC: > Industrial Technology > Energy and Power Engineering > Thermal engineering, heat > Industrial thermal equipment > Heat Transfer Equipment
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