Heat flux, determination

Operating personnel are to be located at a distance from the shield that assures their exposure is less than the heat flux determined by the above equation. In addition, the upper torso of an operator s body shall not be subjected to any visible fire or flame. Flame impingement upon the lower portion of the body may be permitted provided that the heat flux specified above is not exceeded. 

The asymptotic values for the mass loss rate in combustion and flame heat flux determined from the radiation... 

The heat transfer surfaces in a reforming based process can be designed at the maximum heat fluxes determined by the metallurgy of the equipment. The direct oxidation routes suffer from the problem that the heat recovery must take place simultaneously with a strict temperature control in order to ensure maximum selectivity. This makes it difficult to use a simple cooled... 

The calorimetric techniques for measuring heats of mixing two fluids can be classified into their mode of measurement and their principle of heat detection. The isothermal displacement calorimetry will refer to a static mode and flow calorimetry, to a dynamic mode . The principles of heat detection in the following examples will be power compensation or heat flux determination. 

Example 5.17 Estimated Temperature Gradient inside a PEFC Consider a typical PEFC operating at 0.6 V generating around 0.6 W/cm waste heat flux. Determine the expected temperature gradient from the 400-pm-thick cloth DM to the cathode catalyst layer, assuming 50% of the waste heat is removed from the cathode side. 

Temperature and contaminant gradients along the room height and separation stability between the upper and lower zones are influenced by turbulent exchange between these zones. The heat flux density due to turbulent exchange can be determined as ... 

The response factors are characteristic for the layer buildup of the selected wall and are calculated before (by a preprocessor program) or at the beginning ol the simulation. Numerical reasons limit the time step to approximately 10 to 60 min, depending on the thickness and material properties of the wall layers. The method allows the calculation of surface temperatures and heat fluxes bur not the determination of the temperature distribution within the wall. Due to the precalculation of these response factors, the computer time for the simulation might be significantly reduced. 

IFIGI3RE 11.34 Determination of the real evolution of the heat flux at the inside wall surface The temperature condition at the exterior surface is approximated by triangular pulses, and the heat flux response at the interior surface is determined by superposition of the heat flux responses of the individual pulses. 

The room model consists of nodes, which are interconnected by heat exchange paths (Fig. 11.36). The nodes represent either surface temperatures of the individual walls or the zone air temperature. For each node, an energy balance is formulated. From the resulting set of equations, the temperatures and heat fluxes can be determined. 

The area of the fire tube is normally calculated based on a heat flux rate of lO.OOO Btu/hr-ft-. The fire-tube length can be determined from ... 

K = empirically determined constant used as 176 in the range of tj) for bundles q ,3 = maximum heat flux, Btu/ (hr) (fP) i)( = maximum flux physical property factor,... 

Zanker, A. Nomograph Determines Heat Flux of Incipient Boiling, Heating/Piping/Air Conditioning, May (1978) p. 105. 

The Fourier law gives the rate at which heat is transferred by conduction through a substance without mass transfer. This states that the heat flow rate per unit area, or heat flux, is proportional to the temperature gradient in the direction of heat flow. The relationship between heat flux and temperature gradient is characterized by the thermal conductivity which is a property of the substance. It is temperature dependent and is determined experimentally. 

The heat flux radiated from a real surface is less than that from an ideal black body surface at the same temperature. The ratio of real to black body flux is the normal total emissivity. Emissivity, like thermal conductivity, is a property which must be determined experimentally. 

The work of Porter et al. has shown that for copper in phosphoric acid the interfacial temperature was the main factor, and furthermore this was the case for positive or negative heat flux. Activation energies were determined for this system they indicated that concentration polarisation was the rate-determining process, and by adjustment of the diffusion coefficient and viscosity for the temperature at the interface and the application of dimensional group analysis it was found that ... 

Heat-flux corrosion rates can also be determined in plant tests using steam-heated tubular specimens which are weighed or callipered. 

It has been shown that the thermodynamic foundations of plasticity may be considered within the framework of the continuum mechanics of materials with memory. A nonlinear material with memory is defined by a system of constitutive equations in which some state functions such as the stress tension or the internal energy, the heat flux, etc., are determined as functionals of a function which represents the time history of the local configuration of a material particle. 

Infrared thermometry is to identify the maximum permissible temperature of tube alloys, to determine furnace heat flux, scale heat conductivity, and tube heat transfer rates. 

Thus far, the burn-out phenomenon has been discussed mainly in terms of the important system-describing parameters. This approach is preferable, since the system parameters are, in fact, the independent variables and they must uniquely and unequivocally determine the heat flux required to produce burn-out. It can be argued, however, that burn-out, being a local phenomenon, may be described entirely by local parameters of this there can be no dispute. The problem is to find a description of these local parameters that works. Our... 

Whilst the calculation of the radiant heat flux from a gas to an adjoining surface embraces inherent spectral and directional effects, a simplified approach has been developed by Hottel and Manglesdorf 54, which involves the determination of radiation emission from a hemispherical mass of gas of radius L, at temperature 7, ... 

It is a matter speculation as to why such high values of heat flux are obtained with the boiling process. It was once thought that the bubbles themselves were carriers of latent heat which was added to the liquid by their movement. It has now been shown, by determining the numbers of bubbles, that this mechanism would result in the transfer of only a moderate part of the heat that is actually transferred. The current views are that the high flux arises... 

The surface of ultra large-scale integrated circuits (ULSI), from which the heat should be transferred, may be heated by a uniform heat flux, and more often by a non-uniform one. Even in the former case the temperature of the cooled surface is not uniform, but is determined by the heat transfer coefficients along the surface and in the span wise direction. 

Fig. 5.46 A plot of the experimentally determined heat transfer coefficient as a function of the superficial gas velocity and the gas Reynolds number. The liquid mass fluxes are 78.6 and 290 kg/m s, the heat fluxes are 20 and 33 kW/m and the pressure ranges from 140 to 200 kPa. Reprinted from Bao et al. (2000) with permission...

The quasi-one-dimensional model of laminar flow in a heated capillary is presented. In the frame of this model the effect of channel size, initial temperature of the working fluid, wall heat flux and gravity on two-phase capillary flow is studied. It is shown that hydrodynamical and thermal characteristics of laminar flow in a heated capillary are determined by the physical properties of the liquid and its vapor, as well as the heat flux on the wall. 

The maximum possible heat flux, which corresponds to the maximum allowed wall temperatures is estimated. This maximum wall heat flux is determined by the difference between the permissible wall temperature and the vapor temperature in the outlet cross-section, which is a function of the Reynolds and Nusselt numbers. 

Two-phase flows in micro-channels with an evaporating meniscus, which separates the liquid and vapor regions, have been considered by Khrustalev and Faghri (1996) and Peles et al. (1998, 2000). In the latter a quasi-one-dimensional model was used to analyze the thermohydrodynamic characteristics of the flow in a heated capillary, with a distinct interface. This model takes into account the multi-stage character of the process, as well as the effect of capillary, friction and gravity forces on the flow development. The theoretical and experimental studies of the steady forced flow in a micro-channel with evaporating meniscus were carried out by Peles et al. (2001). These studies revealed the effect of a number of dimensionless parameters such as the Peclet and Jacob numbers, dimensionless heat transfer flux, etc., on the velocity, temperature and pressure distributions in the liquid and vapor regions. The structure of flow in heated micro-channels is determined by a number of factors the physical properties of fluid, its velocity, heat flux on... 

The thermohydrodynamic characteristics of the flow in the heated micro-channels depend on the following factors the heat flux on the wall, which determines the intensity of the vaporization, the location of the meniscus, the difference between the inlet and outlet pressures, the capillary, and mass and friction forces which act on the liquid and vapor. 

The position of the meniscus within the micro-channel defines the type of temperature distribution. In the first case, when the meniscus is near the outlet, the temperature gradient of the vapor region is small. The rate of evaporation is determined mainly by the heat flux in the liquid region. Therefore, the necessary condition of the evaporation consists of the existence of the region (near the meniscus), where the water is overheated (its temperature is higher than the temperature of boiling). The heat losses to the inlet tank cause the existence of the temperature maximum. 

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