Thermodynamic modeling of thermosyphons and heat pipes

Abstract

A model capable of predicting the thermodynamic state of the working fluid and its related properties inside a thermosyphon and a heat pipe is proposed. The model theoretically analyzes the entropy changes of various thermodynamic processes and determines the possible locations of the state points on a temperature-specific entropy (T-s) and a specific enthalpy-specific entropy (h-s) diagram at each stage of the thermodynamic cycle. The analysis reveals that the working fluid enters the condenser in a superheated state, while it enters the evaporator in a subcooled state, irrespective of the operating conditions. Analytical expressions are derived to predict the changes in the temperature, pressure, specific volume, entropy, and enthalpy during each thermodynamic process, along with expressions for estimating entropy generation. The effects of varying input heating power (Qiṅ), the fill ratio, the device aspect ratio, and the device inclination angle (θ) on the working fluid behavior are examined, revealing that they affect the thermodynamic state of the working fluid during operation. The conclusion drawn in the existing heat pipe literature that the operating parameters only influence the thermal resistance of thermosyphons and heat pipes is, therefore, incomplete. The geometric and the operational parameters influence the state of the working fluid at each stage of the thermodynamic cycle. The present thermodynamic model, in conjunction with existing heat pipe theory, completely describes thermosyphon and heat pipe operation under any given set of conditions.