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In this study a semi-passive pulse thermal loop (PTL) was designed and experimentally validated. It provides improved heat transfer over passive systems such as the loop heat pipe in the moderate to high heat flux range and can be a sustainable alternative to active systems as it does not require an electric pump. This work details the components of the engineering prototype and characterizes their performance through the application of compressible and two-phase flow theory. A custom LabVIEW application was utilized for data acquisition and control. During operation with refrigerant R-134a the system was shown to be robust under a range of heat loads from 100 W to 800 W. Operation was achieved with driving pressure differentials ranging from 3 bar to 12 bar and pulse frequencies ranging from 0.42 Hz to 0.08 Hz. A smaller pressure differential and an increased pulse frequency results in improved heat transfer at the boilers. An evolution of the PTL is proposed that incorporates a novel, ejector-based pump-free refrigeration system. The design of the pulse refrigeration system (PRS) features valves at the outlet of two PTL-like boilers that are alternately actuated to direct pulses of refrigerant through an ejector. This is intended to entrain and raise the pressure of a secondary stream of refrigerant from the cooling loop, thereby replacing the compressor in a conventional vapor-compression cycle. The PRS is therefore characterized by transient flow through the ejector. An experimental prototype has been constructed which is able to operate as a conventional PTL when the cooling section is bypassed, although full operation of the refrigeration loop remains to be demonstrated. The design of the ejector is carried out using a one-dimensional model implemented in MATLAB that accounts for compressibility effects with NIST REFPROP vapor data sub-routines. The model enables the analysis of ejector performance in response to a transient pressure wave at the primary inlet. The high driving pressures provided by the PTL permit operation in a micro-gravity environment with minimal power consumption. Like the PTL, the proposed PRS is therefore well suited to terrestrial and aerospace applications where it could be driven by waste heat from electronics or solar thermal energy. As a novel semi-passive thermal management system, it will require complex control of the valves. Further analysis of the transient thermodynamic cycle is necessary in order to characterize and effect successful operation of the PRS. / Thesis (M.Sc.)-University of KwaZulu-Natal, Durban, 2013.

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The analytical theory of the “passive thermal design of spacecraft" can be divided into two parts. The first part is concerned with the description of the radiant heat transfer to spacecraft external surfaces. The second part is concerned with calculating temperature over a spacecraft when the radiant heat incident, on the spacecraft's wall, is known. The first part, the calculation of the heat incident on a spacecraft's external surfaces, has been investigated in the literature. References one, two, and three are examples of such papers. Unfortunately, the results of auch papers are either numerical or else too specialized to be of general interest for the analytical study of the thermal design of spacecraft. The second part, the calculation of temperatures over a spacecraft when the incident radiant heat is known, is also dealt with in the literature. References four and five are examples of such papers. The heat flow, occurring in the walls of spacecraft, is nonlinear because of thermal radiation and few exact solutions are known. This problem is usually attacked by "linearizing'' the nonlinear term or by directly employing power aeries. The solution of the nonlinear heat equation by the linearization process is valid only for small temperature variations. When temperature differences are large, the linearized solutions do not properly account for the nonlinear radiation terms and series error can result. When power series are employed directly to solve the nonlinear heat flow equation, the labor required to solve the time dependent problem is generally excessive because the elementary functions cannot be used efficiently. In this thesis, the radiant heat transferred to spacecraft is found by the use of Fourier series. The resulting solutions are simple and are valid for spacecraft of very general geometry. Heat transfer calculation which previously required extensive integration on electronic computers can be calculated by the results of this thesis with only trivial labor. Also, the results have the advantage of being well suited for use in the solution of the nonlinear heat transfer equation. The problem of heat flow including nonlinear radiation is also attached in this thesis. The method of solution used is closely related to the well known method of successive approximations and allows solution of nonlinear equations which do not have the classical “Small perturbation parameter.” Also, the method of solution used makes good use of the elementary functions so that time dependent problems can be solved without excessive labor. The problems solved in this thesis includes: the temperature time history of a body at uniform temperature but exposed to periodic radiative heating, the temperature time history of a body having nonuniform temperatures and exposed to periodic radiative heating, and finally the problem of linear heat flow with nonlinear boundary conditions. In each case it is shown how linearized solutions neglect the important results of nonlinear radiation heat transfer. / Master of Science

Thermal control in spacecraft will be increasingly important as the spacecraft grows smaller and more compact. Such spacecraft with low thermal mass will have to be designed to retain or reject heat more efficiently. The passive smart radiation device (SRD) is a new type of thermal control material for spacecraft. Current space thermal control systems require heaters with an additional power penalty to maintain spacecraft temperatures during cold swings. Because its emissivity can be changed without electrical instruments or mechanical part, the use of SRD decreases the request of spacecraft power budget. The (SRD) based on VO2 films is one of the most important structures of the functional thermal control surface, being lighter, more advanced and without a moving devices. A large portion of the heat exchange between an object in space and the environment is performed throughout radiation, which is in turn determined by the object surface properties. The modulation device is coated on the spacecraft surface and thus provides a thermal window that can adapt to the changing conditions in orbit. VO2 is well known to have a temperature driven metal to insulator transition ≈ 68ᴼC accompanying a transformation of crystallographic structure, from monoclinic (M-phase, semiconductor) at temperature below 68ᴼC to tetragonal (R-phase, metal) at temperature above 68ᴼC. This transition temperature is accompanied by an increase of infrared reflectivity and a decrease of infrared emissivity with increasing temperature. This flexibility makes VO2 potentially interesting for optical, electrical, and electro-optical switches devices, and as window for energy efficiency buildings applications. This study reports on effect of thickness on VO2 as well as the effect of proton irradiation on VO2 for active smart radiation device (SRD) application. VO2 was deposited on mica by Pulsed laser deposition techniques. The thickness of the film was varied by varying the deposition time. To characterize VO2 the following techniques were performed: XRD, AFM, SEM, TEM, XPS, RBS, RAMAN and transport measurements for optical properties. The effect of proton irradiation was observed using the SEM, where the change in structure, from crystal grains to rods, was observed. / Physics / M.Sc. (Physics)

Thermal control in spacecraft will be increasingly important as the spacecraft grows smaller and more compact. Such spacecraft with low thermal mass will have to be designed to retain or reject heat more efficiently. The passive smart radiation device (SRD) is a new type of thermal control material for spacecraft. Current space thermal control systems require heaters with an additional power penalty to maintain spacecraft temperatures during cold swings. Because its emissivity can be changed without electrical instruments or mechanical part, the use of SRD decreases the request of spacecraft power budget. The (SRD) based on VO2 films is one of the most important structures of the functional thermal control surface, being lighter, more advanced and without a moving devices. A large portion of the heat exchange between an object in space and the environment is performed throughout radiation, which is in turn determined by the object surface properties. The modulation device is coated on the spacecraft surface and thus provides a thermal window that can adapt to the changing conditions in orbit. VO2 is well known to have a temperature driven metal to insulator transition ≈ 68ᴼC accompanying a transformation of crystallographic structure, from monoclinic (M-phase, semiconductor) at temperature below 68ᴼC to tetragonal (R-phase, metal) at temperature above 68ᴼC. This transition temperature is accompanied by an increase of infrared reflectivity and a decrease of infrared emissivity with increasing temperature. This flexibility makes VO2 potentially interesting for optical, electrical, and electro-optical switches devices, and as window for energy efficiency buildings applications. This study reports on effect of thickness on VO2 as well as the effect of proton irradiation on VO2 for active smart radiation device (SRD) application. VO2 was deposited on mica by Pulsed laser deposition techniques. The thickness of the film was varied by varying the deposition time. To characterize VO2 the following techniques were performed: XRD, AFM, SEM, TEM, XPS, RBS, RAMAN and transport measurements for optical properties. The effect of proton irradiation was observed using the SEM, where the change in structure, from crystal grains to rods, was observed. / Physics / M.Sc. (Physics)