MATHEMATICAL MODELING OF BENT AMMONIA-ALUMINUM AXIAL GROOVED HEAT PIPES FOR SPACECRAFT APPLICATIONS

A mathematical model for the prediction of the heat transfer characteristics of aluminum-ammonia axial grooved heat pipes with bends is presented. The proposed model accounts for the effects of liquid-vapor interfacial shear stress, the axial variation of the radius of the liquid-vapor interface, and the liquid-solid contact angle between the working fluid and the heat pipe body. The effect of bending is implemented by accounting for the loss of pressure head in both the liquid and the vapor phases of the working fluid due to the presence of the bend. This pressure head loss changes the mass flow rates and the distribution of the working fluid across the length of the heat pipe, which in turn affects the heat transfer characteristics of the heat pipe. The model is validated using experimental data from past studies. A total of 63 different variations of the heat pipe's operating temperature along with the bend angle are studied for this analysis. Based on the observations, a decrease in value is observed in the maximum heat transfer rate (30% decrease in the Q<sub>max</sub>) and the effective thermal resistance (0.2% decrease in the R<sub>Total</sub>) as the bend angle is increased from 0° to 180° at a -20°C operating temperature. Plausible reasons for these trends are discussed by analyzing the variations observed in the working fluid physical properties along the length of the heat pipe during its steady state.