Model of the solutes transfer during osmotic dehydration of vegetal matrices: a proposal

Osmotic dehydration of apple was modeled considering the mechanisms involved in the solutes transfer within the plant matrix: impregnation and diffusion. The model mathematical writing includes the impregnation layer thickness, the diffusion thickness, a water bulk flow and the convective resistance. Apple cylinders were dehydrated at 30, 50 ° C and 30, 50 ° Brix and a motion of 0.15 m/s. The Reynolds number was of 250 for 30°C-30°Bx and 500 for 50°C-50°Bx. Schmidt numbers was of 4000 for 30 ° C-30 ° Bx and 4200 for 50 ° C-50 ° Bx.Keywords: transfer; solute; impregnation; osmotic dehydration.

Analysis of Galinstan-Based Microgap Cooling Enhancement Using Structured Surfaces

Analyses of microchannel and microgap cooling show that galinstan, a recently developed nontoxic liquid metal that melts at −19 °C, may be more effective than water for direct liquid cooling of electronics. The thermal conductivity of galinstan is nearly 28 times that of water. However, since the volumetric specific heat of galinstan is about half that of water and its viscosity is 2.5 times that of water, caloric, rather than convective, resistance is dominant. We analytically investigate the effect of using structured surfaces (SSs) to reduce the overall thermal resistance of galinstan-based microgap cooling in the laminar flow regime. Significantly, the high surface tension of galinstan, i.e., 7 times that of water, implies that it can be stable in the nonwetting Cassie state at the requisite pressure differences for driving flow through microgaps. The flow over the SS encounters a limited liquid–solid contact area and a low viscosity gas layer interposed between the channel walls and galinstan. Consequent reductions in friction factor result in decreased caloric resistance, but accompanying reductions in Nusselt number increase convective resistance. These are accounted for by expressions in the literature for apparent hydrodynamic and thermal slip. We develop a dimensionless expression to evaluate the tradeoff between the pressure stability of the liquid–solid–gas system and hydrodynamic slip. We also consider secondary effects including entrance effects and temperature dependence of thermophysical properties. Results show that the addition of SSs enhances heat transfer.

Analysis of Galinstan-Based Microgap Cooling Enhancement Using Structured Surfaces

Analyses of conventional microchannel and microgap cooling show that galinstan, a recently developed non-toxic liquid metal that melts at −19°C, may be more effective than water for high flux thermal management applications. This is because its thermal conductivity is nearly 28 times that of water. However, since the specific heat per unit volume of galinstan is about half that of water and its viscosity is 2.5 times that of water, caloric, rather than convective, resistance is dominant. We analytically investigate the effect of using microgaps that incorporate structured surfaces to ascertain their efficacy in reducing overall thermal resistance of galinstan-based thermal management in the laminar flow regime. Significantly, the high surface tension of galinstan (10 times that of water) implies that it can remain in the non-wetting Cassie state at the requisite pressure differences for driving flow through microchannels and microgaps. The flow over the structured surface encounters a limited liquid/solid contact area and a low viscosity gas layer interposed between the channel walls and galinstan. Consequent reductions in friction factor result in decreased caloric resistance and reductions in Nusselt number produce an increase in convective resistance. These are accounted for by recently developed expressions in the literature for hydrodynamic and thermal slip.

Bubble Injection to Enhance Heat Transfer in Microchannel Heat Sinks

Liquid cooling is an efficient way to remove heat fluxes with magnitude of 1 to 10,000 W/cm2. One limitation of current single-phase microchannel heat sinks is the relatively low Nusselt number, because of laminar flow. In this work, we experimentally investigate how to enhance the Nusselt number in the laminar regime with the periodic injection of non-condensable bubbles in a water-filled array of microchannels in a segmented flow pattern. We designed a polycarbonate heat sink consisting of an array of parallel microchannels with a low ratio of heat to convective resistance, to facilitate the measurement of the Nusselt Number. Our heat transfer and pressure drop measurements are in good agreement with existing correlations, and show that the Nusselt number of a segmented flow is increased by more than a hundred percent over single-phase flow provided the mass velocity is within a given range.

Thermal Management in the Measurement of Metal Hydride Kinetics

Many metal hydride nanopowders are currently being investigated as a potential hydrogen storage media. The kinetic properties of hydrogen absorption in TiCrMn, a metal hydride of interest, are largely unknown. This study will use coupled thermal and kinetic modeling to analyze a combination of novel and well-established techniques which can be used to experimentally determine these parameters. Since these measurements must be taken at isothermal conditions and the metal hydride absorption reaction is highly exothermal, specific thermal considerations must be made in these models. Typical instruments available for kinetics measurements suspend the samples in a small chamber, effectively thermally isolating them from the cooling or heating system designed to control sample temperature. The design modeled herein will eliminate that convective resistance layer, thereby increasing the amount of heat that can be rapidly diffused out of the sample. Additionally, an electronically controlled active temperature control system will be modeled as a method of maintaining “quasi-isothermal” conditions in the metal hydride during measurements.

Experimental Investigation of the Heat Transfer Characteristics of Aluminum-Foam Heat Sinks With Restricted Flow Outlet

This study investigates the heat transfer characteristics of aluminum-foam heat sinks with restricted flow outlets under impinging-jet flow conditions. An annular flow-restricting mask is used to control the height of the flow outlet of the aluminum foam sink, forcing the cooling air to reach the heat-generation surface. The enhanced heat transfer characteristics of aluminum-foam heat sinks using these flow-restricting masks are measured experimentally in this work. The effects of porosity, pore density and length of sample, air velocity, and flow outlet height on the heat transfer characteristics of aluminum-foam heat sinks are investigated. Results show that the effect of the flow outlet height is stronger than that of the pore density, porosity, or height of the aluminum heat sinks studied in this work. A general correlation between the Nusselt number and the Reynolds number based on the equivalent spherical diameter of the aluminum foam is obtained for 32 samples of aluminum-foam heat sinks with different sample heights (20–40mm), pore densities (5–40ppi(pore∕inch)), porosities (0.87–0.96), and flow outlet heights (5–40mm). It should be noted that, based on the measured velocity profile, the increase of the Nusselt number of the aluminum-foam heat sink with the decrease in the flow outlet height is caused by the reduced convective resistance at the solid-gas interface through the increased velocity near the heat-generation surface. The reduction in flow outlet height increases the local thermal nonequilibrium condition near the heat-generation surface.

Simplified Network Based Modeling of Cold Plate in a CFD Environment

Thermal management of high power electronics for aerospace applications frequently utilizes liquid or air-cooled cold plates with embedded fin cores. These commonly used cold plates use fin assemblies with small flow passages and large area enhancements to achieve high levels of heat transfer performance. The design of this type of cold plate is well documented in the literature, with the most common methodology utilizing “f” and “j” test data as a function of Reynolds Number. This paper presents a technique termed “network modeling” that simplifies the modeling of cold plate features within a CFD model. This technique greatly reduces model size and CPU time needed for solutions. In addition, it is inherently accurate because it allows test data to be incorporated into the model. Simplification of the performance of coldplate features within a system level CFD thermal model is a great advantage, as modeling these small coldplate features is a tedious task and often unnecessary. The methodology presented uses a convective resistance network with mass flow links and convective links to describe the overall thermal behavior of the coldplate. This simplified network model can be used within a detailed thermal model of the electronics assembly to provide an accurate simplification of the coldplate performance for temperature and heat flow prediction. Since the network technique simplifies the flow boundary conditions, the detailed thermal model can contain as much internal details of an electronics assembly as desired, while still keeping the overall model size manageable and CPU times minimal. This network-based method of modeling coldplate should be very accurate because it is based upon established test data of “f” and “j” as the basis of the model. This network method has significant advantages over the other methods of heat exchanger simplification such as coarse mesh, effective thermal conductivity, source-sink, etc. This paper describes the creation of such a network, integration in an ICEPAK thermal model, discussion of the advantages, and results.