Heat Transfer to a Particle Exposed to a Rarefied Plasma with a Great Temperature Gradient

A kinetic-theory analysis is presented concerning the heat transfer from a rarefied plasma to a spherical particle for the extreme case of free-molecule regime and thin plasma sheath. A great temperature gradient is assumed to exist in the plasma, and thus a non-Maxwellian velocity distribution function is employed for each of the gas species. Analytical results show that the existence of a temperature gradient in the plasma causes a nonuniform distribution of the local heat flux density on the sphere surface, while the total heat flux to the whole particle is independent of the temperature gradient. The nonuniformity of the local heat flux distributioln is small even for the case with a temperature gradient as great as 10~6 K/m, but it may significantly enhance the thermophoretic force on an evaporating particle. Heat transfer is mainly caused by atoms at low gas temperatures with negligible ionization degree, while it can be attributed to ions and electrons at high plasma temperatures.     

Condensation Heat Transfer Inside a Tube in a Microgravity Environment

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Heat Transfer and Fluid Flow over a Single Disk in a Fluid Rotating as a Rigid Body

Laminar heat transfer problem is analyzed for a disk rotating with the angular speed ωin a co-rotating fluid (with the angular speed Ω). The fluid is swirled in accordance with a forced-vortex law, it rotates as a solid body at β= Ω/ω= const. Radial variation of the disk's surface temperature follows a power law. An exact numerical solution of the problem is obtained basing on the self-similar profiles of the local temperature of fluid, its static pressure and velocity components. Numerical computations were done at the Prandtl numbers Pr = 1(?)0.71. It is shown that with increasing βboth radial and tangential components of shear stresses decrease, and to zero value at β= 1. Nusselt number is practically constant at β= 0(?) 0.3 (and even has a point of a maximum in this region); Nu decrease noticeably for larger βvalues.     

Natural Convection Heat Transfer From a Hot Rectangular and a Square Corrugated Plate to a Cold Flat Plate

IntroductionReduction of heat loss from the absorber plate of asolar collector through the cover plates improvescollector efficiency. Therefore, the natllral convectionheat loss across air layers bounded by tWo parallel platesis of special interest to the designers of solar collectors.Most of the investigations on heat transfer in aconfined space have been cAned out with parallel platesin horizontal and inclined positions. Hollands, et al.[l]experimentally investigated the heat trallsferchara…     

Simulation of a Turbulent Impinging Jet into a Layer of Porous Material Using a Two–Energy Equation Model

This article presents numerical results for a turbulent jet impinging against a flat plane covered with a layer of permeable and thermally conducting material. Distinct energy equations are considered for the solid porous material attached to the wall and for the fluid that impinges on it. Parameters such as Reynolds number, porosity, permeability, thickness, and thermal conductivity of the porous layer are varied in order to analyze their effects on the local distribution of Nu. The macroscopic equations for mass, momentum, and energy are obtained based on volume-average concept. The numerical technique employed for discretizing the governing equations was the control volume method with a boundary-fitted nonorthogonal coordinate system. The SIMPLE algorithm was used to handle the pressure-velocity coupling. Results indicate that inclusion of a porous layer eliminates the peak in Nu at the stagnation region. For highly porous and highly permeable material, simulations indicate that the integral heat flux from the wall is enhanced when a thermally conducting porous material is attached to the surface.     

Heat Transfer in a Forced Wall Jet on a Heated Rough Surface

Many stUdies of wall Jets on smooth, flat and curveds~es haVe been examined over the past fifty years.Glauertll] realized the first comprehensive study of thewall jet. A survey by Launder and Rodiln summarizedmost of the known results on tulbulent jet flows on flatsauce. The stUdy of the effect of significant rouglmesscaused by the deposition of pericles on a heated wallsubmited tO acoustic vibrations, can be interesting foraPPlications in problems of cooling in engine brineblades, for exa…     

Is a fully-developed and non-isothermal flow possible in a vertical pipe?

The results of a linear-stability analysis of the fully-developed flow in a heated vertical pipe are presented. They confirm the experimental observations that flow in a heated vertical pipe is supercritically unstable. The bifurcated new equilibrium laminar flow is likely to be a double spiral flow. Mixing induced by this spiral flow can cause a substantial increase in the heat-transfer rate and even delay transition to turbulence, as has been observed experimentally.     

COSSOR—a transient simulation program for a solid absorption solar refrigerator

An algorithm for the computer simulation of a solid absorption solar refrigerator, COSSOR, is presented. The algorithm is based on a detailed transient mathematical model of the system. It is a sequential modular program which consists of a relatively short main program and about 50 subprograms which are called by name by the main program or other subprograms. The program is coded in QBASIC, is highly flexible, accurate and fast, and has been tested on a wide variety of personal computers. Predictions of the refrigerator performance obtained using COSSOR compare very well with experimental data over a wide range of operating conditions. It is therefore a useful tool for computer aided design, performance prediction and analysis of the refrigerator, and may find applications in other solar thermal systems. Typical predicted results for some stations are given.     

Online humidification diagnosis of a PEMFC using a static DC–DC converter

This paper deals with the online checking of the humidification of a Proton Exchange Membrane Fuel Cell (PEMFC). Indeed, drying or flooding can decrease the performance of the PEMFC and even lead to its destruction. An online humidification diagnosis can allow a real-time control. A good indicator of the membrane humidification state is its internal resistance. As known, the membrane ionic conductivity increases with the membrane water content. This resistance can be calculated at high frequency by dividing the voltage variation by the current variation. The proposed scheme makes use of measurements of current and voltage ripples coming from the association of a static DC–DC converter and the fuel cell. The experiment thus consists in computing the internal resistance in wet and dry conditions.     

Heat transfer in a reaction–diffusion system with a moving heat source

A general formulation is presented for a moving boundary problem in which heat is generated at the boundary due to an exothermic reaction involving a species which diffuses into a dispersed phase from an external medium of finite volume. The speed of the moving boundary is prescribed based on the solution of the mass diffusion problem and an analysis is presented of the thermal dynamics of the system. The set of equations describing heat transport leads to a Green’s function type problem with time dependent boundary conditions and the Galerkin finite element method is employed to develop a numerical solution. Transformations are introduced to freeze the moving boundary and partition the domain for ease of computation, and an iterative scheme is defined to satisfy the heat flux jump boundary condition and match the temperature field across the moving boundary. The numerical results are used to set the limits of applicability of an analytical perturbation solution. Essential aspects of thermal dynamics in the system are described and parametric regions resulting in a local temperature hot spot are delineated. Computed contour plots describing thermal evolution are presented for different combinations of parameter values. These may be of utility in the prediction of thermal development, for control and avoidance of hot spot formation, and in physical parameter estimation.     

The temperature response of a metallic rod near a “steam explosion”

This communication presents the results of a series of experiments designed to investigate anomalous thermal responses of metallic bars subjected to rapid local cooling. Heating to 150 °C and subsequent rapid cooling of one end of a copper bar resulted in a temporal increase in the temperature of the other end of the bar of about 4 °C. Similar behavior was observed for other metallic bars, namely steel, aluminum, and brass. We will refer to this effect as “Heating Through Cooling”. The physics of this phenomena appears to be similar to the process called a “steam explosion”.     

Performance improvement for a simple cycle gas turbine GENSET––a retrofitting example

Due to the serious power shortage in Taiwan, many simple cycle gas turbine generation sets (GENSETs) that were originally designated to serve as peak load units are forced to operate continuously during the entire summer season. The retrofitting projects have been seriously considered to convert these GENSETs (which have the advantage of fast startup, but suffer from low power output and thermal efficiency at high ambient temperature) into more advanced cycle units with higher efficiency and higher output. Among many proven technologies, such as inlet air cooling, intercooling, regeneration, reheating and steam-injection gas turbine (STIG) etc., STIG is found to be one of the most effective in boosting both the output capacity and thermal efficiency. The results from computer simulation indicated that the retrofitting of existing GE Frame 6B simple cycle unit into STIG cycle can boost the output from about 38 to 50 MW, while the generation efficiency can be increased from about 30% to 40%. Besides, the power output of STIG cycle is less sensitive to ambient temperature than that of simple cycle. NO_x reduction to less then 25 ppm (when LNG is used) and operating flexibility under variable heat demand could be achieved.     

Coherent Structures Generated by a Circular Jet Issuing into a Cross Laminar Boundary Layer

Visualisations by LASER topographies and velocity measurements by LDV have allowed the study of the flow resulting from the interaction between a circular jet and a cross boundary layer. This type of flow is dominated by the presence of many complex vortices that come from the recombining of the vorticity created in the injection tube and that created along the chamber floor.     

Influences of a Kind of Groove-blade on the Flow Field in a Compressor Cascade

This paper presents an experimental investigation of effects of a kind of streamwise-grooved blades on the flowfield in a compressor cascade.The flow field downstream the cascade and the boundary layer on the suctionsurface were measured using a mini 5-hole pressure probe at different incidence angles.The flow field in thegroove cascade was compared with that in the smooth cascade.The measurement results indicate that:(1)thegroove surface can restrain the development of the boundary layer on the suction surface;(2)the grooves canrestrain the radial migration of the low-energy fluids in the boundary layer on the suction surface;(3)the grooveblades can reduce total pressure loss and flow blockage in the cascade at the incidence angles of 0°,5°and 8°;(4)the maximum benefit of 8.6% loss reduction was obtained at the incidence angle of 5° while negative benefit of-3.0% loss reduction occurred at the incidence angle of-5°.     

The Two-Dimensional Supersonic Flow and Mixing with a Perpendicular Injection in a Scramjet Combustor

A numerical investigation has been performed on supersonic mixing of hydrogen with air in a Scramjet (Supersonic Combustion Ramjet) combustor and its flame holding capability by solving Two-Dimensional full Navier-Stokes equations. The main flow is air entering through a finite width of inlet and gaseous hydrogen is injected perpendicularly from the side wall. An explicit Harten-Yee Non-MUSCL Modified-flux-type TVD scheme has been used to solve the system of equations, and a zero-equation algebraic turbulence model to calculate the eddy viscosity coefficient. In this study the enhancement of mixing and good flame holding capability of a supersonic combustor have been investigated by varying the distance of injector position from left boundary keeping constant the backward-facing step height and other calculation parameters. The results show that the configuration for small distance of injector position has high mixing efficiency but the upstream recirculation can not evolved properly which is an importa     

Optimal sizing of a solar–biogas-based cooking system for a cluster of villages

Energy supplies particularly in remote and far-flung rural areas are in pathetic situation. Leave aside other needs, most of the rural communities still use wood as a source of energy for cooking. Burning of wood is not only an inefficient method, but also hazardousness for the person working on the stove. People have been working for cleaner and efficient means of cooking for decades. Solar cooker- and biogas-based cook stoves are two of the successfully implemented technologies in this area. Although solar cooker requires no maintenance, the initial investment is quite high for a cluster of villages. In addition to this, the intermittency involved in solar energy makes this an unreliable source.

In this paper, a cluster of villages of Narendra Nagar block of Tehri Garhwal district of Uttarakhand, India, has been studied in terms of their thermal requirements. The potential of solar energy and biomass energy has been estimated. An integrated solar–biogas system has been proposed to satisfy this cooking demand. To obtain the optimal sizes of solar cooker and biogas generator, MATLAB codes have been developed. It was found that this system is more economical and much reliable than the other two cases.     

Experimental performance of a solar air heater with a “V” corrugated absorber

The performance of a “V” corrugated-plate solar air heater was investigated experimentally. The collector was tilted at 20° in summer and 40° in winter for maximum heat collection during the period of maximum solar irradiance. The collector performance parameters were evaluated during clear days in winter and summer. The mass flow rate of air was varied from 0.016 to 0.0385 kg/s·m², and an air outlet temperature of 70°C was obtained in summer at midday with an average collector efficiency of 42%. The contribution of solar energy from the collector for winter space heating was estimated.     

Optimization Study of a Coanda Ejector

The Coanda effect has long been employed in the aerospace applications to improve the performances of variousdevices.This effect is the ability of a flow to follow a curved contour without separation and has well been util-ized in ejectors where a high speed jet of fluid emerges from a nozzle in the ejector body, follows a curved sur-face and drags the secondary flow into the ejector.In Coanda ejectors,the secondary flow is dragged in the ejec-tor due to the primary flow momentum. The transfer of momentum from the primary flow to the secondary flowtakes place through turbulent mixing and viscous effects.The secondary flow is then dragged by turbulent shearforce of the ejector while being mixed with the primary flow by the persistence of a large turbulent intensitythroughout the ejector.The performance of a Coanda ejector is studied mainly based on how well it drags thesecondary flow and the amount of mixing between the two flows at the ejector exit.The aim of the present studyis to investigate the influence of various geometric parameters and pressure ratios on the Coanda ejector per-formance.The effect of various factors,such as,the pressure ratio, primary nozzle and ejector configurations onthe system performance has been evaluated based on a performance parameter defined elsewhere.The perform-ance of the Coanda ejector strongly depends on the primary nozzle configuration and the pressure ratio.The mix-ing layer growth plays a major role in optimizing the performance of the Coanda ejector as it decides the ratio ofsecondary mass flow rate to primary mass flow rate and the mixing length.