Capital investment reduction, exergetic efficiency improvement and material compatibility issues have been identified as the primary techno-economic challenges associated, with the near-term development and deployment of thermal energy storage (TES) in commercial-scale concentrating solar power plants. Three TES techniques have gained attention in the solar energy research community as possible candidates to reduce the cost of solar-generated electricity, namely (1) sensible heat storage, (2) latent heat (tank filled with phase change materials (PCMs) or encapsulated PCMs packed in a vessel) and (3) thermochemical storage. Among these the PCM macro-encapsulation approach seems to be one of the most-promising methods because of its potential to develop more effective energy exchange, reduce the cost associated with the tank and increase the exergetic efficiency. However, the technological barriers to this approach arise from the encapsulation techniques used to create a durable capsule, as well as an assessment of the fundamental thermal energy transport mechanisms during the phase change. A comprehensive study of the energy exchange interactions and induced fluid flow during melting and solidification of a confined storage medium is reported in this investigation from a theoretical perspective. Emphasis has been placed on the thermal characterization of a single constituent storage module rather than an entire storage system, in order to, precisely capture the energy exchange contributions of all the fundamental heat transfer mechanisms during the phase change processes. Two-dimensional, axisymmetric, transient equations for mass, momentum and energy conservation have been solved numerically by the finite volume scheme. Initially, the interaction between conduction and natural convection energy transport modes, in the absence of thermal radiation, is investigated for solar power applications at temperatures (300 - 400°). Later, participating thermal radiation within the storage medium has been included in order to extend the conventional natural convection-dominated model and to analyze its influence on the melting and freezing dynamics at elevated temperatures (800 - 850°). A parametric analysis has been performed in order to ascertain the effects of the controlling parameters on the melting/freezing rates and the total and radiative heat transfer rates at the inner surface of the shell. The results show that the presence of thermal radiation enhances the melting and solidification processes. Finally, a simplified model of the packed bed heat exchanger with multiple spherical capsules filled with the storage medium and positioned in a vertical array inside a cylindrical container is analyzed and numerically solved. The influence of the inlet mass flow rate, inner shell surface emissivity and PCM attenuation coefficient on the melting dynamics of the PCM has been analyzed and quantified.

Numerical simulation of mold filling in low pressure die casting is considered in this study. The physical model includes modeling of free surface flow, heat transfer with phase change, surface tension, natural convection together with effect of trapped air in the mold. The governing equations are discretized by control volume finite difference method. The pressure field is computed by two-step projection method and the free surface is tracked by PLIC-VOF method. Water analog model is used for the validation purpose. Good agreement between numerical and experimental results is observed which supports the feasibility of the presented method.

Impulse technology is a process in which water is removedfrom a wet paper web by the combined action of mechanicalpressure and intense heat. This results in increased dewateringrates, increased smoothness on the roll side of the sheet, andincreased density. Although the potential benefits of impulsepressing have been debated over the past thirty years, itsindustrial acceptance has been prevented by web delamination,which is defined as a reduction in the z-directional strengthof paper. This thesis deals with the mechanism of heat transfer withphase change during impulse pressing of wet paper. The resultsof four complementary experimental studies suggest that littleor no steam is formed in an impulse nip prior to the point ofmaximum applied load. As the nip is unloaded and the hydraulicpressure decreases, hot liquid water flashes to steam. Weadvance the argument that the force expressed upon flashing canbe used to displace liquid water, in a mechanism similar tothat originally proposed by Wahren. Additionally, modelexperiments performed in a novel experimental facility suggestthat the strength of flashing-assisted displacement dewateringcan be maximized by controlling the direction of steam venting.If this solution could be exploited in a commercially viableimpulse press, delamination would cease to be an issue ofconcern. The thesis includes a study of the web structure ofdelaminated paper. Here, we characterized delaminated paper bythe changes in transverse permeability and cross-sectionalsolidity profiles measured as a function of pressingtemperature. We found no evidence that wet pressing and impulsepressing induced stratification in non-delaminated sheets andconcluded that the parabolic solidity profiles observed weredue to capillary forces present during drying. Further, thepermeability of mechanically compressed never-dried samples wasfound to be essentially constant for pressing temperatureslower than the atmospheric boiling point of water and toincrease significantly at higher pressing temperatures. Wepropose this to be a result of damage to the cell wall materialdue to flashing of hot liquid water in the fiber walls andlumina. Finally, we present a method and an apparatus formeasurement of the thermal properties of water-saturated paperwebs at temperatures and pressures of interest for commercialhigh-intensity processes. After validation, the method wassuccessfully applied to measure the thermal conductivity,thermal diffusivity and volumetric heat capacity ofwater-saturated blotter paper as functions of temperature andsolids content. Here, we found that the thermal conductivityincreased with solids content in the range from 30%\ to 55%,which is in conflict with the commonly stated assumptions of adecreasing trend. We propose that this discrepancy could be dueto the thermal conductivity of air-free fibers wetted byunpressable water only, being significantly different from thatof dry cellulose.

The molten pool shape and thermocapillary convection during melting or welding of metals or alloys are self-consistently predicted from scale analysis. Determination of the molten pool shape and transport variables is crucial due to its close relationship with the strength and properties of the fusion zone. In this work, surface tension coefficient is considered to be negative, indicating an outward surface flow, whereas high Prandtl number represents a thinner thickness of the thermal boundary layer than that of momentum boundary layer. Since Marangoni number is usually very high, the domain of scaling is divided into the hot, intermediate and cold corner regions, boundary layers on the solid-liquid interface and ahead of the melting front. The results find that the width and depth of the pool, peak and secondary surface velocity, and maximum temperatures in the hot and cold corner regions can be explicitly and separately determined as functions of working variables or Marangoni, Prandtl, Peclet, Stefan, and beam power numbers. The scaled results agree with numerical data, different combinations among scaled equations, and available experimental data.

The present research work aims to develop a new heat transfer fluid by combining multiwalled carbon nanotubes (MWCNT) and microencapsulated phase change materials (MPCMs). Stable nanofluids have been prepared using different sizes of multiwalled carbon nanotubes and their properties like thermal conductivity and viscosity have been measured. Microencapsulated phase change material slurries containing microcapsules of octadecane have been purchased from Thies Technology Inc. Tests have been conducted to determine the durability and viscosity of the MPCM slurries. Heat transfer experiments have been conducted to determine the heat transfer coefficients and pressure drop of the MWCNT nanofluids and MPCM slurries under turbulent flow and constant heat flux conditions. The MPCM slurry and the MWCNT nanofluid have been combined to form a new heat transfer fluid. Heat transfer tests have been conducted to determine the heat transfer coefficient and the pressure drop of the new fluid under turbulent flow and constant heat flux conditions. The potential use of this fluid in convective heat transfer applications has also been discussed. The heat transfer results of the MPCM slurry containing octadecane microcapsules was in good agreement with the published literature. The thermal conductivity enhancement obtained for MWCNTs with diameter (60-100 nm) and length (0.5-40?m) was 8.11%. The maximum percentage enhancement (compared to water) obtained in the heat transfer coefficient of the MWCNT nanofluid was in the range of 20-25%. The blend of MPCMs and MWCNTs was highly viscous and displayed a shear thinning behavior. Due to its high viscosity, the flow became laminar and the heat transfer performance was lowered. It was interesting to observe that the value of the maximum local heat transfer coefficient achieved in the case of the blend (laminar flow), was comparable to that obtained in the case of the MPCM slurry (turbulent flow). The pressure drop of the blend was lower than that of the MWCNT nanofluid.

This thesis presents numerical modelling and analysis of a solar Organic Rankine Cycle (ORC) for electricity generation. A regression based approach is used for the working fluid property calculations. Models of the unit&rsquo / s sub-components (pump, evaporator, expander and condenser) are also established. Steady and transient models are developed and analyzed because the unit is considered to work with stable (i.e. solar + boiler) or variable (i.e. solar only) heat input. The unit&rsquo / s heat exchangers (evaporator and condenser) have been identified as critical for the applicable method of analysis (steady or transient). The considered heat resource into the ORC is in the form of solar heated water, which varies between 80-95 0C at a range of mass flow rates between 2-12 kg/s. Simulation results of steady state operation using the developed model shows a maximum power output of around 40 kW. In the defined operation range / refrigerant mass flow rate, hot water mass flow rate and hot water temperature in the system are identified as critical parameters to optimize the power production and the cycle efficiency. The potential benefit of controlling these critical parameters is demonstrated for reliable ORC operation and optimum power production. It is also seen that simulation of the unit&rsquo / s dynamics using the transient model is imperative when variable heat input is involved, due to the fact that maximum energy recovery is the aim with any given level of heat input.

<p>Impulse technology is a process in which water is removedfrom a wet paper web by the combined action of mechanicalpressure and intense heat. This results in increased dewateringrates, increased smoothness on the roll side of the sheet, andincreased density. Although the potential benefits of impulsepressing have been debated over the past thirty years, itsindustrial acceptance has been prevented by web delamination,which is defined as a reduction in the z-directional strengthof paper.</p><p>This thesis deals with the mechanism of heat transfer withphase change during impulse pressing of wet paper. The resultsof four complementary experimental studies suggest that littleor no steam is formed in an impulse nip prior to the point ofmaximum applied load. As the nip is unloaded and the hydraulicpressure decreases, hot liquid water flashes to steam. Weadvance the argument that the force expressed upon flashing canbe used to displace liquid water, in a mechanism similar tothat originally proposed by Wahren. Additionally, modelexperiments performed in a novel experimental facility suggestthat the strength of flashing-assisted displacement dewateringcan be maximized by controlling the direction of steam venting.If this solution could be exploited in a commercially viableimpulse press, delamination would cease to be an issue ofconcern.</p><p>The thesis includes a study of the web structure ofdelaminated paper. Here, we characterized delaminated paper bythe changes in transverse permeability and cross-sectionalsolidity profiles measured as a function of pressingtemperature. We found no evidence that wet pressing and impulsepressing induced stratification in non-delaminated sheets andconcluded that the parabolic solidity profiles observed weredue to capillary forces present during drying. Further, thepermeability of mechanically compressed never-dried samples wasfound to be essentially constant for pressing temperatureslower than the atmospheric boiling point of water and toincrease significantly at higher pressing temperatures. Wepropose this to be a result of damage to the cell wall materialdue to flashing of hot liquid water in the fiber walls andlumina.</p><p>Finally, we present a method and an apparatus formeasurement of the thermal properties of water-saturated paperwebs at temperatures and pressures of interest for commercialhigh-intensity processes. After validation, the method wassuccessfully applied to measure the thermal conductivity,thermal diffusivity and volumetric heat capacity ofwater-saturated blotter paper as functions of temperature andsolids content. Here, we found that the thermal conductivityincreased with solids content in the range from 30%\ to 55%,which is in conflict with the commonly stated assumptions of adecreasing trend. We propose that this discrepancy could be dueto the thermal conductivity of air-free fibers wetted byunpressable water only, being significantly different from thatof dry cellulose.</p>

Open celled micro and nano foams fabricated from polymers and metals have attracted tremendous attention in the recent past because of their applications in numerous areas such as catalyst carriers, filtration media, ion exchange membranes and tissue engineering scaffolds. In this study open celled polymer micro- and nano foams with controllable pore size and porosity were fabricated via solid state foaming of immiscible blends. The polymer foams were used as templates for fabricating nickel foams using an ethanol based electroless plating process. Thermal conductivity of micro- and nano foams was studied as a function of pore size and porosity using finite element and molecular dynamics based models. The effect of pore size and porosity on performance of phase change material infiltrated metal foams for thermal management was investigated via numerical models. Open celled micro foams were fabricated via solid state foaming of ethylene acrylic acid (EAA) and polystyrene (PS) co-continuous blends. Blending temperature was the main parameters affecting the formation of co-continuous structure. Gas saturation and foaming studies were performed to determine ideal processing conditions for the blend. The results indicated that saturation pressure and foaming temperature were major process parameters determining the porosity of the foamed samples. Open celled polymer templates were obtained by selective extraction of PS phase using dichloromethane (DCM). Foaming resulted in faster extraction of PS and also in a higher porosity. Open celled nano foams were fabricated via solid state foaming of polyetherimide (PEI) and polyethersulfone (PES). The effect of process parameters namely saturation pressure and temperature, desorption time, and foaming temperature and time on porosity and pore size was studied. A high gas concentration and foaming temperature were required to obtain nano pore-sized foams. Throughout the cross section there existed regions with varying pore size and porosity and solid skins at the surface regions of the foam. A solvent surface dissolution process using dimethylformamide (DMF) was employed to access the internal porous structure. Micro- and nano cellular nickel foams were fabricated from EAA and PES templates via electroless plating. The structure of the nickel foams was an inverse of the polymer templates. Ethanol based electroless plating solutions were used to ensure infiltration into the porous structure because of the small pore sizes. Finite element and molecular dynamics based models were developed to predict thermal conductivity of polymer foams as a function of pore size and porosity. Pore sizes ranging from 1 nm to 1 mm were studied. Models were partially validated using experimental data. The results showed that pore size has significant effect on thermal conductivity even for microcellular and conventional foams. When the pore size is reduced to the nanometer scale, the thermal conductivity of the nano foam dramatically reduces and the value could be lower than that of air for certain porosity levels. The extremely low thermal conductivity of polymer nanofoams is possibly due to increased phonon-phonon scattering in the solid phases of the polymer matrix in addition to low thermal conductivity of gas trapped in nano sized pores. Finite element based models were also developed to study the effect of pore size and porosity on performance of phase change material infiltrated metal foams for thermal management applications. The results showed that foams with smaller pore sizes can delay the temperature rise of the heat source for an extended period of time by rapidly dissipating heat in the phase change material. The lower temperatures resulting from the use of a smaller pore size metal foam could significantly increase the lifetime of IC chips. / text

Spray cooling is a technique for achieving large heat fluxes at low surface temperatures by impinging a liquid in droplet form on a heated surface. Heat is removed by droplets spreading across the surface, thus removing heat by evaporation and by an increase in the convective heat transfer coefficient. The addition of nano-sized particles, like aluminum or copper, to water to create a nanofluid could further enhance the spray cooling process. Nanofluids have been shown to have better thermophysical properties when compared to water, like enhanced thermal conductivity. Although droplet size, velocity, impact angle and the roughness of the heated surface are all factors that determine the amount of heat that can be removed, the dominant driving mechanism for heat dissipation by spray cooling is difficult to determine. In the current study, experiments were conducted to compare the enhancement to heat transfer caused by using alumina nanofluids during spray cooling instead of de-ionized water for the same nozzle pressure and distance from the heated surface. The fluids were sprayed on a heated copper surface at a constant distance of 21 mm. Three mass concentrations, 0.1%, 0.5%, and 1.0%, of alumina nanofluids were compared against water at three pressures, 40psi, 45psi, and 50psi. To ensure the suspension of the aluminum oxide nanoparticles during the experiment, the pH level of the nanofluid was altered. The nanofluids showed an enhancement during the single-phase heat transfer and an increase in the critical heat flux (CHF). The spray cooling heat transfer curve shifted to the right for all concentrations investigated, indicating a delay in two-phase heat transfer. The surface roughness of the copper surface was measured before and after spray cooling as a possible cause for the delay.

The diffusion absorption refrigeration (DAR) cycle is a promising technology for fully thermally driven cooling. It is well suited to applications in medicine refrigeration and air-conditioning in off-grid settings. However, design and engineering knowhow for the technology is limited; therefore, system development has historically been an iterative and expensive process. Additionally, conventional system designs require high-grade energy input for operation, and are unsuitable for low-temperature solar- or waste-heat activated applications. In the present effort, component- and system-level DAR engineering analyses are performed. Detailed bubble-pump generator (BPG) component models are developed, and are validated experimentally and with direct simulations. Investigations into the BPG focus on the Taylor flow pattern in the intermediate Bond number regime, which has not yet been thoroughly characterized in the literature, and has numerous industry applications, including nuclear fuel processing and well dewatering. A coupling-fluid heated BPG design is also investigated experimentally for low-source-temperature operation. Phase-change simulation methodologies are developed to rigorously study the continuously developing flow pattern in this BPG configuration. Detailed component-level models are also formulated for all of the other DAR heat and mass exchangers, and are integrated to yield a complete system-level model. Results from these modeling studies are applied to develop a novel fully passive low-source-temperature (110 - 130°C) DAR system that delivers refrigeration grade cooling. This design achieves operation at target conditions through the use of alternate working fluids (NH3-NaSCN-He), the coupling-fluid heated BPG, and a novel absorber configuration. The complete DAR system is demonstrated experimentally, and evaluated over a range of operating conditions. Experimental results are applied to assess and refine component- and system- level models.