Date of Award

12-2014

Document Type

Thesis

Degree Name

Master of Engineering (ME)

Legacy Department

Mechanical Engineering

Advisor

Dr. Jay M. Ochterbeck

Committee Member

Dr. Xiangchun Xuan

Committee Member

Dr. Chenning Tong

Abstract

Heat transfer devices find applications in various aspects of life. Be it residential, commercial or industrial application, efficient heat transfer is a challenge to all. Other than geometric design considerations and wick selection, the optimization of heat transfer in the heat pipe also depends on fluid selection. Heat pipe technology has proven to work efficiently with properly selected thermal fluid, from cryogenic temperatures to very high temperatures. Higher heat transfer ability through small temperature differences makes the heat pipe an efficient technology. Hence, it can be stated that selecting a proper working fluid enhances the heat transfer performance of a heat pipe. For selecting the working fluid, important thermo-physical properties to be considered are density, viscosity, surface tension, latent heat of vaporization and vapor saturation pressure at every working temperature. The operating range of the working fluid starts from the triple point and till the critical point. The performance of the working fluid is not optimum at both ends of the operating range of temperature. At critical temperature, it is impacted by low surface tension and latent heat of vaporization, whereas near the triple point low vapor density and high viscosity affects the performance. One of the first indices for evaluating the performance of the working fluid is called 'Merit Number' This merit number considers a single pressure gradient, i.e. the liquid pressure drop. Later, substantial works have been done to implement the same idea in a system utilizing multiple pressure gradients (losses). In all the methods comparing the merit number of the fluids, the higher the merit number, better is the heat transfer capacity of the pipe. For theoretical calculations and geometrical design considerations, thermo-physical property data of the working fluid at every operating temperature is not available and if available, the reliability of this data is a reason of concern. The present work constitutes of dividing the working fluids into two main categories polar fluids (i.e. ammonia, water and methanol) and nonpolar fluids (i.e. ethane) and thus validating the methods used for formulating these thermo-physical properties as a function of temperature. As per conventional available data (in several reliable resources), these thermo-physical properties are formulated as a polynomial function of the temperature. The main problem though with such formulation is the data reliability outside the specified temperature range. This work tries to formulate such properties as a function of intensive properties and molecular structure of the working fluid. Thereafter the most useful method for thermo-physical property formulation was chosen after calculating the error percentage (relating to the experimental data obtained from various sources) The latter part of this work focuses on the uncertainty of the value about the mean obtained from the methods used and thereafter the percent deviation (between the mean obtained and the experimental data available) which can give the clear idea about the selection of the method for formulating the properties. The last part of this work link the different methods used with the merit number for both liquid and vapor driven heat pipe. This part also includes the error percent and deviation percentage of the capillary limited maximum heat transferring capacity of the heat pipe.

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