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Performance evaluation of a non-adiabatic capillary tube in a transcritical CO2 heat pump cycle.

Author(s) : AGRAWAL N., BHATTACHARYYA S.

Type of article: Article

Summary

A non-adiabatic capillary tube in a transcritical CO2 heat pump cycle has been simulated to investigate the effect of parameters such as gas cooler and evaporator temperature, capillary tube diameter and heat exchanger length on various performance indicators. The homogeneous flow model is employed to simulate two-phase flow in the non-adiabatic capillary tube. Fundamental equations of mass, energy and momentum are solved simultaneously through an iterative process. Single and two-phase heat transfer coefficients are calculated by employing appropriate empirical correlations. Subcritical and supercritical thermodynamic and transport properties of CO2 are calculated employing an in-house precision property code. Lowering evaporator temperature is found to be more effective for heat transfer from the capillary tube compared to the gas cooler temperature. Heat transfer rate variation with respect to gas cooler temperature in case of CO2 is distinctly different compared to conventional refrigerants due to its transcritical nature and is influenced by initial quality, mass flow rate of the refrigerant and the prevailing temperature difference at the gas cooler. Increase in gas cooler temperature causes the heat transfer rate to first increase and then to decrease. Lowering evaporator and gas cooler temperature increases the cooling capacity. Throttling effect decreases rapidly as internal tube diameter becomes larger leading to higher mass flow rate of the refrigerant. Shorter inlet adiabatic capillary length with larger heat exchanger length is better for heat transfer. This study is an attempt to allay the scepticism prevailing in the parlance of CO2 based transcritical systems overemphasising the need for a throttle valve to control the optimum discharge pressure. [Reprinted with permission from Elsevier. Copyright, 2007].

Details

  • Original title: Performance evaluation of a non-adiabatic capillary tube in a transcritical CO2 heat pump cycle.
  • Record ID : 2008-1610
  • Languages: English
  • Source: International Journal of thermal Sciences - vol. 47 - n. 4
  • Publication date: 2008/04

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