No description available.

No description available.

Boiling in microchannels is a very efficient mode of heat transfer with high heat and mass transfer coefficients achieved. Less pumping power is required for two-phase flows than for single-phase liquid flows to achieve a given heat removal. Applications include electronics cooling such as cooling microchips in laptop computers, and process intensification with compact evaporators and heat exchangers. Evaporation of the liquid meniscus is the main contributor to the high heat fluxes achieved due to phase change at thin liquid films in a microchannel. The microscale hydrodynamic motion at the meniscus and the flow boiling heat transfer mechanisms in microchannels are not fully understood and are very different from those in macroscale flows. Flow instability phenomena are noted as the bubble diameter approaches the channel diameter. These instabilities need to be well understood and predicted due to their adverse effects on the heat transfer. A fundamental approach to the study of two-phase flow boiling in microchannels has been carried out. Simultaneous visualisation and hydrodynamic measurements were carried out investigating flow boiling instabilities in microchannels using two different working fluids (n-Pentane and FC-72). Rectangular, borosilicate microchannels of hydraulic diameter range 700-800 μm were used. The novel heating method, via electrical resistance through a transparent, metallic deposit on the microchannel walls, has enabled simultaneous heating and visualisation to be achieved. Images and video sequences have been recorded with both a high-speed camera and an IR camera. Bubble dynamics, bubble confinement and elongated bubble growth have been shown and correlated to the temporal pressure fluctuations. Both periodic and nonperiodic instabilities have been observed during flow boiling in the microchannel. Analysis of the IR images in conjunction with pressure drop readings, have allowed the correlation of the microchannel pressure drop to the wall temperature profile, during flow instabilities. Bubble size is an important parameter when understanding boiling characteristics and the dynamic bubble phenomena. In this thesis it has been demonstrated that the flow passage geometry and microchannel confinement effects have a significant impact on boiling, bubble generation and bubble growth during flow boiling in microchannels.

Cryopreservation is the preservation of biomaterials at extremely low temperatures. It is the only alternative for long-term storage of high quality biomaterials, with applications to biobanking and transplant medicine. Cryopreservation success revolves around the control of ice formation, which is known to be harmful. Ice formation is a path-dependent phenomenon, affected by the thermal history and presence of nucleation promotors. Cryoprotective agents (CPAs) are commonly added to the biomaterial to be preserved, in order to suppress ice formation and inhibit its growth during the cryopreservation protocol. Ice-free cryopreservation can be achieved in large-size systems when the biomaterial is loaded with a high CPA concentration solution and cooled rapidly, in a process that is known as vitrification (vitreous means glassy in Latin). During vitrification, the CPA viscosity increases exponentially with decreasing temperature, while the material is cooled to deep cryogenic temperatures faster than the typical time scale for crystallization. The material can potentially be stored indefinitely at such low temperatures. Large-size vitrification is associated with three competing needs on the CPA concentration. Since the cooling rate at the center of the specimen decreases with the increasing specimen size due to the scaling conductive resistance, higher CPA concentrations may be required to suppress crystallization in larger specimens. Higher CPA concentration generally requires lower cooling rates to avoid ice crystallization. On the other hand, since CPAs are potentially toxic, the lowest possible CPA concentration is required to maintain viability and facilitate functional recovery. The decrease in CPA concentration combined with an increase in cooling rates may intensify thermo-mechanical stress due to non-uniform thermal contraction to the point of structural destruction. Essentially, successful cryopreservation represents the outcome of an optimization problem on the composition and concentration of the CPA cocktail. The work presented in this thesis combines an experimental study on the thermal conductivity of relevant materials, and a theoretical study to identify the effects of the measured values on cryopreservation protocols. The unique contributions presented as the initial stage of the experimental study are: (i) the modification of the cryomacroscope and creation of an experimental program to make thermal conductivity measurements of CPA based on the existing transient hot wire technique, (ii) to develop a protocol for making thermal conductivity measurements during rewarming portion of the cryoprotocol, and (iii), to begin generating a data bank of thermal conductivity of CPA and materials used in cryopreservation. Thermal conductivity measurements are presented for the CPA Dimethyl Sulfoxide (DMSO), over a concentration range of 2M to 10M, in a temperature range of -180°C to 25°C. Samples of 2M to 6M DMSO were found to crystallize at quasi-steady cooling rates, while samples of 7.05 to 10M were found to vitrify. Thermal conductivities of the crystallized and vitrified material reach a tenfold difference at -180°C. The quality of measurements using the presented technique has been verified theoretically by means of finite element analysis (FEA) using the commercial code ANSYS. This experimental study is expanded to the study of thermal conductivity of the CPA cocktail DP6--a mixture of 3M DMSO and 3M propylene glycol, which has drawn significant attention in the cryobiology community in recent times. The unique contributions are the first thermal conductivity measurements reported in literature of the combined effect of DP6 with synthetic ice modulators (SIMs), including 6% 1,3Cyclohexanediol, 6% 2,3Butanediol, and 12% PEG400. Results of this study demonstrate that the thermal conductivity may vary by three fold between the amorphous and crystalline phases of DP6 below the glass transition temperature. Results of this study further demonstrate the ability of SIMs to decrease the extent of crystallization in DP6, even at subcritical cooling and rewarming rates. The accompanying theoretical investigation focuses on cryopreservation in a kidney model, in effort to explore how the thermal history is affected by variations in the measured thermal conductivity. This analysis is based on FEA using the commercial code ANSYS. In particular, the unique contributions of this study are: (i) thermal analysis of a vitrifying rabbit kidney based on an established rabbit-kidney cryopreservation protocol, and (ii), exploring scale-up thermal effects to a human-size organ. This represents a 21-fold increase in organ size. Results indicate that even in the case of the human kidney, cooling rates remain high enough in all parts of the kidney to prevent ice formation at temperatures above -100oC.

Approved for public release; distribution is unlimited / Heat-transfer measurements were made for condensation of R-113 and steam on a smooth tube and on three finned tubes with rectangular shape fins. These tubes had a fin height and width of 1.0 mm and spacings of 0.25, 1.5, and 4.0 mm (tubes A, B, and C respectively) . Data were taken by increasing the vapor velocity from 0.4 to 1.9 m/s for R-113 and 4.8 to 31.3 m/s for steam. For both fluids, the improvement of the condensing heat-transfer coefficient with vapor velocity was smaller for the finned tubes than for the smooth tube. For R-113, the smooth tube experienced a 32 percent improvement with vapor velocity, where the finned tubes (tubes A, B and C respectively) experienced improvements of only 0, 5 and 10 percent. For steam, the smooth tube experienced a 62 percent improvement, whereas the finned tubes (tubes A, B, and C respectively) experienced improvements of only 31, 11, and 9 percent. These test results show that, although finned tubes can provide significant heat transfer enhancement over smooth tubes at low vapor velocities, the degree of enhancement becomes smaller as vapor velocity increases. / CBT-8603582 (NSF) / http://archive.org/details/effectofvaporvel00hopk / National Science Foundation / Lieutenant Commander, United States Navy

It is well established that rarefied flows cannot be properly described by traditional hydrodynamics, namely the Navier-Stokes equations for gas flows, and the Fourier’s law for heat transfer. Considering the significant advancement in miniaturization of electronic devices, where dimensions become comparable with the mean free path of the flow, the It is well established that rarefied flows cannot be properly described by traditional hydrodynamics, namely the Navier-Stokes equations for gas flows, and the Fourier's law for heat transfer. Considering the significant advancement in miniaturization of electronic devices, where dimensions become comparable with the mean free path of the flow, the study of rarefied flows is extremely important. This dissertation includes two main parts. First, we look into the heat transport in solids when the mean free path for phonons are comparable with the length scale of the flow. A set of macroscopic moment equations for heat transport in solids are derived to extend the validity of Fourier's law beyond the hydrodynamics regime. These equations are derived such that they remain valid at room temperature, where the MEMS devices usually work. The system of moment equations for heat transport is then employed to model the thermal grating experiment, recently conducted on a silicon wafer. It turns out that at room temperature, where the experiment was conducted, phonons with high mean free path significantly contribute to the heat transport. These low frequency phonons are not considered in the classical theory, which leads to failure of the Fourier's law in describing the thermal grating experiment. In contrast, the system of moment equations successfully predict the deviation from the classical theory in the experiment, and suggest the importance of considering both low and high frequency phonons at room temperature to capture the experimental results. In the second part of this study, we look into the gas-surface interactions for conventional gas dynamics when the gas flow is rarefied. An extension to the well-known Maxwell boundary conditions for gas-surface interactions are obtained by considering velocity dependency in the reflection kernel from the surface. This extension improves the Maxwell boundary conditions by providing an extra free parameter that can be fitted to the experimental data for thermal transpiration effect in non-equilibrium flows. The velocity dependent Maxwell boundary conditions are derived for the Direct Simulation Monte Carlo (DSMC) method and the regularized 13-moment (R13) equations for conventional gas dynamics. Then, a thermal cavity is considered to test and study the effect of these boundary conditions on the flow formation in the slip and early transition regime. It turns out that using velocity dependent boundary conditions allows us to change the size and direction of the thermal transpiration force, which leads to marked changes in the balance of transpiration forces and thermal stresses in the flow. / Graduate

Calcined coke is the best material for making carbon anodes for smelting of alumina to aluminum. Calcining is an energy intensive industry and a significant amount of heat is wasted in the calcining process. Efficiently managing this energy resource is tied to the profit margin and survivability of a calcining plant. 3-D computational models are developed using FLUENT to simulate the calcining process inside the long slender kiln. Simplified models are employed to simulate the moving petocke bed with a uniform distribution of moisture evaporation, devolatilization, and coke fines entrainment rate with a conjugate radiation-convection-conduction calculation. The results show the 3-D behavior of the flow, the reaction inside the kiln, heat transfer and the effect of the tertiary air on coke bed heat transfer. The ultimate goals are to reduce energy consumption, recover waste-heat, increase thermal efficiency, and increase the product yield.

Two mathematical models (a one and a three-dimensional) were adopted to study, numerically, the thermal hydrodynamic behavior of flow inside a single cooling channel of a Nuclear Thermal Rocket (NTR) engine. The first model assumes the flow in the cooling channel to be one-dimensional, unsteady, compressible, turbulent, and subsonic. The working fluid (GH2) is assumed to be compressible. The governing equations of the 1-D model are discretized using a second order accurate finite difference scheme. Also, a commercial CFD code is used to study the same problem. Numerical experiments, using both codes, simulated the flow and heat transfer in a cooling channel of the reactor. The steady state predictions of both models were compared to the existing experimental results and it is concluded that both models successfully predict the steady state fluid temperature distribution in the NTR cooling channel.

In this thesis, combustion processes in a laboratory-scale methane based low pressure rocket motor (LPRM) is studied experimentally and numerically. Experiments are conducted to measure flame temperatures and chamber temperature and pressure. Single reaction-four species reacting flow of gaseous methane and gaseous oxygen in the combustion chamber is also simulated numerically using a commercial CFD solver based on 2-D, steady-state, viscous, turbulent and compressible flow assumptions. LPRM geometry is simplified to several configurations, i.e. Channel and Combustion Chamber with Nozzle and FWD. Flow in a Bunsen burner is simulated inside Channel geometry in order to validate the reaction model. Grid independence study is also conducted for reacting as well as non-reacting flows. Numerical model is calibrated based on experimental results. Results of the computational model are found in a good agreement with the experimental data after calibrating specific heats of the products. Parametric study is conducted in order to investigate the effects of different mass flow rates and chamber pressures on flow and combustion characteristics of a LPRM to provide insight to future studies.

The HAbitability, Brine Irradiation and Temperature Package (HABIT) instrument of the ExoMars Surface Platform will investigate the present day habitability of Mars at the near surface environment. This instrument includes three Atmospheric Temperature Sensor's (ATS's) which are similar to the ones previously used on the Rover Environmental Monitoring Station (REMS) of the Mars Science Laboratory (MSL) rover, that has now been operating on Mars for more than four years. The ATS of REMS is only used to provide the air temperature, however on HABIT it will be used furthermore to provide information about winds and heat transfer at the surface of Mars. The retrieval method needs to be further investigated and validated. This master thesis is aimed at three goals: 1) the development and testing of an Earth Scaled Atmospheric Temperature Sensor (ESATS) to test the retrieval concept; 2) the validation with other Earth-based standard wind sensing technologies under outdoors uncontrolled conditions; and 3) the analysis of the existing observations of the ATS of REMS on Mars to get a better understanding of its expected future performance on HABIT once it operates on Mars. The ESATS is an up-scaled semi-autonomous prototype version of an ATS which consists of a rod of different size and material to those that are used on REMS and will be used on HABIT. The rod shall be heated from the base where it is attached to. The temperature profile shall be measured at three different measurement points. All these temperatures are different from the one of the atmosphere to which the rod is exposed to. The temperature profile along the rod changes depending on the air temperature, air density and the wind speed because of the convective heat transfer. A preliminar analysis is used to define what is the ideal length of the rod, and what is the material that is best adapted for this experimental prototype. Since the air density is needed to retrieve the wind speed, the pressure will be monitored as well. In parallel, a second wind measuring technique based on the dynamic pressure changes detected in a Pitot tube is used as control. The measuring campaign is subdivided in several stages: 1) The first part will take place in a laboratory, where the system is exposed to static conditions with no heating and no wind, which means that there is no forced convection caused by wind. In this setup all sensors are calibrated against one another and with help of a reference resistor the temperature sensors are also calibrated to 273.15K. Additionally different Operational Amplifiers (OpAmps) will be used to observe how the noise level is affecting the measurements, so that the best one will be used in the end. 2) Furthermore, the best position to place the intermediate temperature sensor is investigated by testing one of the rods with different locations of the middle temperature sensor. 3) Next, also within the laboratory environment, the different rods are used to obtain the temperature profile and retrieve the air temperature and heat transfer values, solving the equations that describe the heat transfer problem under static conditions. 4) The second part of the measurement campaign will take place outdoors, where the ESATS is exposed to forced convection due to wind. In this setting first the influence of the Sun on the system is measured, as it is important to know, if the measurement can be performed when the illumination conditions change. 5) Next, the system is tested with the 50cm rod in long term tests with the reference measurement of a commercial weather station (HOBO) next to it. With the data obtained the convective heat transfer method is used and the retrieved wind speed is compared to the one received from the HOBO. Finally, to get a better understanding of its expected future performance on HABIT once it operates on Mars, the data of the ATS of REMS is used to perform the wind speed retrieval for Mars and to compare it with the data received from the REMS wind sensor. It is only operating during daytime and has still difficulties to retrieve a precise wind speed. The measurement campaign has given several information about ATS in general. First it was decided to place the temperature sensor in the middle at 1/4 of the rod length, which is optimal for the retrieval process and which is also coincident with the one chosen for HABIT and REMS. The measurements in the laboratory are providing good and constant temperature profiles with the chosen setup which correspond with the one expected from the equations that describe the heat transfer problem in a long rod. On the other hand, it is not possible to calculate a valid ambient temperature for the short rods, which is because of an overheated boundary layer around the rods due to the heating. For this reason, it is recommended to use the longest rod in the lab. For outside testing the influence of the Sun could be confirmed and was affecting the measurements of the copper rod. During the time span where the prototype was in the Sun, it was not possible to get any reasonable results. The next measurement campaign was defined in a shadowed area with diffuse light only. Finally, the tests of exposure to dynamic changes over time are in excellent agreement with the ones provided by the HOBO station and can even give a better resolution and sensitivity to small changes of wind magnitude. This prototype has confirmed experimentally, that under Earth conditions, this method can be used to retrieve the wind speed. Finally, the Martian data of the REMS ATS are analyzed and the comparisons suggest that the method is sensitive to wind changes on Mars as well, and shows better time and magnitude resolution than the existing REMS wind sensor. This confirms that this method can be successfully used for the HABIT sensor. In this work the methodology that shall be used on HABIT to retrieve the convective heat transfer and wind on the surface of Mars is validated for Earth outdoors conditions. It is recommended to verify the obtained results with tests in a wind tunnel and to see how the system will behave with a higher heating and different rod materials. Furthermore, the setup should be tested in a way which makes it possible to determine the different directions of the wind.