Convective coefficient

Nusselt Equation for Various Geometries Natural-convection coefficients for various bodies may be predicted from Eq. (5-32). The various numerical values of 7 andm have been determined experimen-... 

Radiation differs from conduction and convection not only in mathematical structure but in its much higher sensitivity to temperature. It is of dominating importance in furnaces because of their temperature, and in ciyogenic insulation because of the vacuum existing between particles. The temperature at which it accounts for roughly half of the total heat loss from a surface in air depends on such factors as surface emissivity and the convection coefficient. For pipes in free convection, this is room temperature for fine wires of low emissivity it is above red heat. Gases at combustion-chamber temperatures lose more than 90 percent of their energy by radiation from the carbon dioxide, water vapor, and particulate matter. 

This is equivalent to a convection coefficient of 44057278, or 15.85 W7(m")(K) which is of the order of magnitude expected of the convection coefficient itself. Radiation rapidly becomes dominant as the system temperature rises. 

Pressure drop due to hydrostatic head can be calculated from hquid holdup B.]. For nonfoaming dilute aqueous solutions, R] can be estimated from f i = 1/[1 + 2.5(V/E)(pi/pJ ]. Liquid holdup, which represents the ratio of liqmd-only velocity to actual hquid velocity, also appears to be the principal determinant of the convective coefficient in the boiling zone (Dengler, Sc.D. thesis, MIT, 1952). In other words, the convective coefficient is that calciilated from Eq. (5-50) by using the liquid-only velocity divided by in the Reynolds number. Nucleate boiling augments conveclive heat transfer, primarily when AT s are high and the convective coefficient is low [Chen, Ind Eng. Chem. Process Des. Dev., 5, 322 (1966)]. 

Equation (12-47) assumes that all heat sources are at the same temperature and that the convection coefficients to the evaporating surface and to the unwetted portions of the tray are equal. When radiation occurs from a source at a different temperature, the radiation coefficient can be corrected to the same basis by multiplying by the ratio (t — f()/(U i ), where t, t, and t are the diying-gas, evaporating-surface, and radiator temperatures respectively. 

The forced-convection coefficient hfc is calculated assuming that the liquid phase is flowing in the conduit alone. 

Chen s method was developed from experimental data on forced convective boiling in vertical tubes. It can be applied, with caution, to forced convective boiling in horizontal tubes, and annular conduits (concentric pipes). Butterworth (1977) suggests that, in the absence of more reliable methods, it may be used to estimate the heat-transfer coefficient for forced convective boiling in cross-flow over tube bundles using a suitable cross-flow correlation to predict the forced-convection coefficient. Shah s method was based on data for flow in horizontal and vertical tubes and annuli. 

Hereby I is the integral of the temperature difference (PCM temperature, T, minus ambient temperature, Too) vs. time. It considers constant specific heat for the substance and tube, and constant external convection coefficient. 

A solid is to be dried from 1 kg water/kg dry solids to 0.01 kg water/kg dry solids in a tray dryer consisting of a single tier of 50 trays, each 0.02 m deep and 0.7 m square completely filled with wet material. The mean air temperature is 350 K and the relative humidity across the trays may be taken as constant at 10 per cent. The mean air velocity is 2.0 m/s and the convective coefficient of heat transfer is given by ... 

The liquid mass transfer coefficient (or convection coefficient) is a flow-dependent parameter, which can be evaluated from several correlations found in the related literature. 

In writing the energy balance (Equation 5.17) we slipped another fast one by the reader by assuming h to be the same as ho, the heat transfer coefficient from the liquid film-air film interface to the air. If a significant resistance is associated with the liquid film, h is better expressed by the overall heat transfer coefficient (U), which consists of both he andhL. Note that hL is the convection coefficient from the liquid film to the interface [7], When appreciable liquid film resistance exists, our energy balance should be written as... 

Pleat exchange coefficients Thermal convective coefficient Thermal radiative emissivity 5Wm-2 K-1 0.3... 

The heat flux at any given surface is comprised of both convective and radiative components. The convective heat flux between the gas and solid is obtained through the use of a convective coefficient ... 

Object submerged in a fluid. Consider an object with a characteristic length L and a thermal conductivity k that is submerged in a fluid of constant temperature Too and convection coefficient h (see Fig.5.9). If a heat balance is made on the surface of the object, it must be equivalent to the heat by conduction, i.e.,... 

Estimated by the convective coefficient of single-phase gas flow [Sleicher and Rouse, 1975]... 

At this point we should remark that the installation of fins on a heat-transfer surface will not necessarily increase the heat-transfer rate. If the value of h, the convection coefficient, is large, as it is with high-velocity fluids or boiling liquids, the fin may produce a reduction in heat transfer because the conduction resistance then represents a larger impediment to the heat flow than the convection resistance. To illustrate the point, consider a stainless-steel pin fin which has k = 16 W/m °C, L = 10 cm, d = I cm and which is exposed to... 

A rod containing uniform heat sources per unit volume q is connected to two temperatures as shown in the accompanying figure. The rod is also exposed to an environment with convection coefficient h and temperature Tm. Obtain an expression for the temperature distribution in the rod. 

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