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MODELING OF TRANSIENT FLOW THROUGH CAPILLARY TUBE- SUCTION LINE HEAT EXCHANGERS

DOI: 10.1615/IHTC13.p18.280
12 pages

A. L. Seixlack
Department of Mechanical Engineering, Sao Paulo State University, Ilha Solteira, Brazil

R. A. Navas
Department of Mechanical Engineering, Sao Paulo State University, Ilha Solteira, Brazil

Abstract

This work presents a numerical model to simulate the unsteady refrigerant flow through capillary tube-suction line heat exchangers. Capillary tubes are commonly used as expansion devices in small refrigeration and air conditioning systems. Capillary tube and suction line are considered straight and horizontal with constant inner diameter, and it is assumed one-dimensional flow. The flow through capillary tube is divided in two distinct regions: a region of subcooled liquid flow and a region of saturated two-phase flow. The homogeneous model is employed for the two-phase flow region. The fundamental equations governing the flow through capillary tube-suction line heat exchanger are derived from the mass conservation, momentum and energy conservation laws. Due to compressibility of the flow in the two-phase region, critical or choked flow condition is generally found in capillary tubes. The system of governing equations is solved using a modified finite volume method. The model allows prediction, in steady and unsteady states, refrigerant mass flow rates, pressure, quality, refrigerant and wall temperatures distributions along the tubes, as a function of the heat exchanger geometry and operating conditions. Experimental data from the literature for steady flow are compared and discussed with numerical results. The discrepancies between measured and calculated mass flow rate have been found to be about 8.6 %, for concentric heat exchangers, and 5.7 % for lateral heat exchangers. Additionally comparisons between the transient and quasi-steady models are presented and the values for the numerically evaluated mass flow rates have differed by approximately 2 %.

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Measurement of fluid temperature with an arrangement of three thermocouples