1 Chaer 4 Heat Transfer and Its Applications.ppt

Unsteady-state conduction For steady one-dimensional heat flow, Eq.(A) may be written Thermal conductivity k The proportionality constant k is a physical property of the substance. It , like the Newtonian viscosity μ, is one of the so-called transport properties of material. For small ranges of temperature, k may be considered constant. k varies over a wide range. They are highest for metals and lowest for finely powdered materials from which air has been evacuated. For glass and most nonporous materials, the thermal conductivities are much lower, from about 0.35 to 3.5W/m.oC. Gases have the smallest thermal conductivities, with values as low as 0.007W/m.oC. 4.2.2 Steady-state conduction 1. Flat slab For the simplest case of steady-state conduction, consider a flat slab like that shown in following Fig. assuming Since in steady state there can be neither accumulation nor depletion of heat within the slab, q is constant along the path of heat flow. Since the only variables in Eq.(4.2-1) are x and T, direct integration gives Equation (4.2-8) can be written in the form 2. Compound resistance in series Consider a flat wall constructed of a series of layers, as shown in the Fig. Then, if ΔT is the total temperature drop across the entire wall The rate of heat flow through several resistances in series is analogous to the current flowing through several electric resistances in series. 3. Heat flow through a cylinder Consider the hollow cylinder represented by the figure. The thermal conductivity of the material of which the cylinder is made is k. The rate of heat flow through an arbitrary cylinder, concentric with the main cylinder, is given The ratio of the logarithmic mean to the arithmetic mean is a function of ro/ri ΔT= ΔTA+ ΔTB+ ΔTC In heat flow through a series of layers the overall thermal resistance equals the sum of the individual resistances R=RA+RB+RC 堂垂凸资确缺住鸭瘫票信林砒建莆竭痴竞揩吸馅复依薛绰两保揽照天蒸原1 Chaper 4 Heat Transfer and Its Applications1 Chaper 4 Heat