Unit thermal convective conductance

The heat transfer rate, q, is taken as positive in the direction wall-to-fluid so that it will have the same sign as(Tw -T/) and h will always, therefore, be positive. A number of names have been applied to h including convective heat transfer coefficient , heat transfer coefficient , film coefficient , film conductance , and unit thermal convective conductance . The heat transfer coefficient, h, has the units W/m2-K or, since its definition only involves temperature differences, W/m2oC, in the SI system of units. In the imperial system of units, h has the units Btu/ft2-hr-°F. 

This is the rate of heat transfer from a surface to the surrounding air (or fluid) due to conduction convection and radiation. It is generally used only in still-air conditions and when the temperature difference between surface and ambient is of the order of 30 K. It is obtained by dividing the thermal transmission per unit area in watts per square meter by the temperature difference between the surface and the surrounding air. It is expressed as W/nf K. 

The mechanisms described above tell us how heat travels in systems, but we are also interested in its rate of transfer. The most common way to describe the heat transfer rate is through the use of thermal conductivity coefficients, which define how quickly heat will travel per unit length (or area for convection processes). Every material has a characteristic thermal conductivity coefficient. Metals have high thermal conductivities, while polymers generally exhibit low thermal conductivities. One interesting application of thermal conductivity is the utilization of calcium carbonate in blown film processing. Calcium carbonate is added to a polyethylene resin to increase the heat transfer rate from the melt to the air surrounding the bubble. Without the calcium carbonate, the resin cools much more slowly and production rates are decreased. 

Heat can be transferred by conduction, convection, or radiation and/or combinations thereof. Heat transfer within a homogeneous solid or a perfectly stagnant fluid in the absence of convection and radiation takes place solely by conduction. According to Fourier s law, the rate of heat conduction along the y-axis per unit area perpendicular to the y-axis (i.e., the heat flux q, expressed as W in - or kcal m 2 h ) will vary in proportion to the temperature gradient in the y direction, dt/dy (°C m or K m ), and also to an intensive material property called heat or thermal conductivity k (W m K or kcal h m °C ). Thus,... 

Thermal Conductivity. The most frequently investigated thermophysical property of textiles is thermal conductance, or U, the heat flux without convection transfer (usually expressed as calories/meters2 x seconds x °C), or its reciprocal, thermal resistance. Thermal conductivity, or k, is thermal conductance normalized with respect to the heat flux per unit degree temperature across unit thickness of the material (expressed in calories/ meters x seconds x °C). Many studies have demonstrated that thermal conductance primarily depends on fabric thickness and air present in the material however, the conductivity of air accounts for the greater part of the conductivity values observed (1 2,... 

FIG. 5-4 Thermal circuit for Example 1. Steady-state conduction in a furnace wall with heat losses from the outside surface by convection (hc) and radiation (hR) to the surroundings at temperature Tsur. The thermal conductivities kD, kg, and ks are constant, and there are no sources in the wall. The heat flux q has units of W/m2. 

SOLUTION A steam pipe covered with glass v ool insulation is subjected to convection on its surfaces. The rate of heat transfer per unit length and the temperature drops across the pipe and the insulation are to be determined. Assumplians I Heat transfer is steady since there is no indication of any change with time. 2 Heat transfer is one-dimenslonal since there is thermal symmetry about the centerline and no variation in the axial direction. 3 Thermal conductivities are constant. 4 The thermal contact resistance at the Interface is negligible. 

The parameter a was called the thermal diffusivity by Kelvin and the thermometric conductivity by Ma.xwell. but Kelvin s expression has been generally adopted. It measures the change in temperature that would be produced in unit volume of the substance by the quantity of heal that flows in unit time across unit area of a layer of the sub.stance of unit thickness with unit temperature difference between its faces [3]. It is the parameter that determines the non-steady state temperature distribution in the absence of heat generation and convection and is therefore essential for transient heat flow calculations. 

A surface heat transfer coefficient h can be defined as the quantity of heat flowing per unit time normal to the surface across unit area of the interface with unit temperature difference across the interface. When there is no resistance to heat flow across the interface, h is infinite. The heat transfer coefficient can be compared with the conductivity the conductivity relates the heat flux to the temperature gradient the surface heat transfer coefficient relates the heat flux to a temperature difference across an unknowm distance. Some theoretical work has been done on this subject [8], but since it is rarely possible to achieve in practice the boundary conditions assumed in the mathematical formulation, it is better to regard it as an empirical factor to be determined experimentally. Some typical values are given in Table 2. Cuthbert [9] has suggested that values greater than about 6000 W/m K can be regarded as infinite. The spread of values in the Table is caused by mold pressure and by different fluid velocities. Heat loss by natural convection also depends on whether the sample is vertical or horizontal. Hall et al. [10] have discussed the effect of a finite heat transfer coefficient on thermal conductivity measurement. 

Prandtl number (Pr, Npd n. A dimensionless group important in the analysis of convection heat transfer, defined as (in consistent units) Cpp/k, where Cp is the specific heat of a fluid at constant pressures, p the viscosity, and k is the thermal conductivity. The Prandtl number is also the ratio of the kinematic viscosity to the thermal diffusivity see both entries). 

Liquid metals are sometimes used as a heat-transfer fluid in cases where a fluid is needed over a wide temperature range at relatively low pressures. Liquid metals are often used in nuclear reactors and have high heat-transfer coefficients as well as a high heat capacity per unit volume. The high heat-transfer coefficients are due to the very high thermal conductivities and, hence, low Prandtl numbers. In liquid metals in pipes, the heat transfer by conduction is very important in the entire turbulent core because of the high thermal conductivity and is often more important than the convection effects. 

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