Thermal resistance convection

For purposes of physical reasoning, convection and radiation heat flow can be viewed as being represented by (nonlinear) thermal resistances, convective 9 and radiational 9r, respectively,... 

Above this size, the flow of air over the condenser surface will be by forced convection, i.e. fans. The high thermal resistance of the boundary layer on the air side of the heat exchanger leads to the use, in all but the very smallest condensers, of an extended surface. This takes the form of plate fins mechanically bonded onto the refrigerant tubes in most commercial patterns. The ratio of outside to inside surface will be between 5 1 and 10 1. 

The temperature difference, T1 - T2, is the driving force in this case, the quantity 1 jh S (= R), is known as the thermal resistance for convective heat transfer, and h is called the surface coefficient. 

The total thermal resistance comes from a combination of forced convection from the body and thermal conduction through the skin ... 

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,... 

The guarded hot plate is a standard instrument for measuring the relative thermal resistance of textiles as heat flows from a heated plate in contact with the textile and dissipates into still air at a lower ambient temperature via radiation, conduction, and convection. By design, it minimizes errors due to edge heat losses and validates the total quantity of heat flowing through the specimens. Convection and surface radiation can be controlled by use of a hood (2j+). Simpler devices such as the Reeves warmth tester and a chamois-covered copper cylinder also measure thermal... 

There are few studies on instruments that measure thermal resistance of textiles while varying convection and/or air velocity. An early example of these devices employed a refrigeration unit and measured the thermal efficiency or resistance of textiles at air velocities up to 13 mph (31). Later studies measured the thermal -esi stance of knits under free and forced convection (32, 33) some also used bicalorimeters to simulate the shape of the textile during actual wear (3U). 

A node like that shown in Table 3-2d has both x and y increments equal to 1.0 cm. The convection boundary condition is at 50 C and h = 60 W/m2 - °C. The solid material is stainless steel (18% Cr, 8% Ni). Using the thermal resistance and capacitance formulation for a transient analysis write the nodal equation for this node and determine the maximum allowable time increment. 

In this problem the water-side convection coefficient is the main controlling factor because h is so large for a condensation process. In fact, the outside thermal resistance is smaller than the conduction resistance of the steel. The approximate relative magnitudes of the resistances are... 

We stait this chapter with one-dimensional steady heat conduction in a plane wall, a cylinder, and a sphere, and develop relations for thennal resistances in these geometries. We also develop thermal resistance relations for convection and radiation conditions at the boundaries. Wc apply this concept to heat conduction problems in multilayer plane wails, cylinders, and spheres and generalize it to systems that involve heat transfer in two or three dimensions. We also discuss the thermal contact resislance and the overall heat transfer coefficient and develop relations for the critical radius of insulation for a cylinder and a sphere. Finally, we discuss steady heat transfer from finned surfaces and some complex geometries commonly encountered in practice through the use of conduction shape factors. 

A surface exposed to the surrounding air involves convection and radiation simultaneously, and the total heat transfer at the surface is determined by adding (or subtracting, if in the opposite direction) the radiation and convection components. The convection and radiation resistances are parallel to each other, as shown in Pig. 3-5, and may cause some complication in the thermal resistance network. When 7 s T, the radiation effect can properly be accounted for by replacing A in the convection resistance relation by... 

FIGURE 3-9 The thermal resistance network for heat tran.sfer through a two-layer plane wall subjected to convection on botir sides. 

Properties The thermal conductivity is given to be ft = 0.78 W/m K. Analysis This problem involves conduction through the glass window and convection at its surfaces, and can best be handled by making use of the thermal resistance concept and dravring the thermal resistance network, as shown in Fig. 3-12. Noting that the area of the window is 71 - 0.8 m X 1.5 m = 1.2 m, the individual resistances are evaluated from their definitions to be... 

Analysis The contact area between the case and the plate is given to be 8 cm, and the plate area for each transistor is 100 cm. The thermal resistance network of this problem consists of three resistances in scries (interface, plate, and convection), which are determined to be... 

Now consider steady one-dimensional heat transfer through a cylindrical or spherical layer that is exposed to convection on boili sides to fluids at temperatures and T 2 with heat transfer coefftcients /t, and h, respectively, as shown in Fig. 3-25. The thermal resistance network in this case consists of one conduction and two convection resistances in series, just like the one for the plane wall, and the rate of heat transfer under steady conditions can be expressed as... 

FIGURE 3-25 The thermal resistance network for a cylindrical (or spherical) shell subjected to convection from both the inner and the outer sides. 

The thermal resistance network for heal transfer through a three-layered composite cylinder subjected to convection on both sides. 

The thermal resistance concept can also be used for other geometries, provided that the proper conduction resistances and the proper surface areas in convection resistances are used. 

Analysis The thermal resistance network for this problem involves four resistances in series and is given in Fig. 3-29. Taking L = 1 m, the areas of the surfaces exposed to convection are determined to be... 

An insulated cylindrical pipe exposed to convection from the outer surface and the thermal resistance network associated with it. 

The thermal resistance network for this problem involves a conduction resistance for the plastic cover and a convection resistance for the outer surface in series, as shown in Fig. 3-32. The values of these tv/o resistances are... 

Combined natural convection and radiation thermal resistance of various heat sinks used in the cooling of electronic devices between the beat sink and the surroundings. All fins are made of aluminum 6063T-5, are black anodized, and are 76 mm (3 in) long. ... 

The sensitive electronic circuitry of a power transistor at the junction is protested by its case, which is a rigid metal enclosure. Heat transfer characteristics of a power transistor are usually specified by the manufacturer in terms of the case-to-ambient thermal resistance, which accounts for both Ihe natural convection and radiation heat transfers. 

Discussion This transistor can be used at higher power levels by attaching it to a heat sink (which lowers the thermal resistance by increasing the heat transfer surface area, as discussed in the next example) or by using a fan (vrhich lowers the thermal resistance by increasing the convection heal transfer coefficient). 

C Consider a surface of area A at which the convection and radiation heat transfer coefficients are and respectively. Explain how you would determine (a) the single equivalent heat transfer coefficient, and (b) the equivalent thermal resistance. Assume the medium and the surrounding surfaces are at the same temperature. 

The Biot number can be thought of as die ratio of (n) The conduction thermal resistance to the convective thermal resistance. 

The convective thermal resistance to the conduction thermal resistance. 

Analysis The thermal resistance network for this problem involves four resistances in series and is given in Fig. 7-40. The inner radius of the pipe is r-j = 0.8 cm and the outer radius of the pipe and thus the inner radius of the insulation is rj = 1.0 cm. Letting ra represent the outer radius of the insulation, the areas of the surfaces exposed to convection for an L = 1-m-long section of the pipe become... 

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