Cylinders heat transfer coefficient

Even the good heat transfer conditions turned out to be false, however, if the correlation derived for single cylinders by McAdams (1954) were extrapolated to Rep < 100. Nelson and Galloway (1975) pointed out that at low Reynolds numbers the real heat transfer coefficient could be four... 

Figure 9.26. Distribution of the film heat transfer coefficient round a cylinder with flow normal to the axis for...

The differential equation describing the temperature distribution as a function of time and space is subject to several constraints that control the final temperature function. Heat loss from the exterior of the barrel was by natural convection, so a heat transfer coefficient correlation (2) was used for convection from horizontal cylinders. The ends of the cylinder were assumed to be insulated. The equations describing these conditions are ... 

The rate of heat loss from a large insulated cylinder (see Figure E3.3), for which the insulation thickness is much smaller than the cylinder diameter and the inside heat transfer coefficient is very large, can be approximated by the formula... 

At high Re and Ma in the free-molecule regime, transfer rates for spheres have been calculated by Sauer (S4). These results, together with others for cylinders and plates, have been summarized by Schaaf and Chambre (Sll). The particles are subject to aerodynamic heating and the heat transfer coefficients are based upon the difference between the particle surface temperature and the recovery temperature (see standard aerodynamics texts). In the transitional region, the semiempirical result of Kavanau (K2),... 

If the wall is that of hollow cylinder with radii r, and rc>, the overall heat transfer coefficient based on the outside surface is... 

Figure 1736. Effective thermal conductivity and wall heat transfer coefficient of packed beds. Re = dpG/fi, dp = 6Vp/Ap, s -porosity, (a) Effective thermal conductivity in terms of particle Reynolds number. Most of the investigations were with air of approx. kf = 0.026, so that in general k elk f = 38.5k [Froment, Adv. Chem. Ser. 109, (1970)]. (b) Heat transfer coefficient at the wall. Recommendations for L/dp above 50 by Doraiswamy and Sharma are line H for cylinders, line J for spheres, (c) Correlation of Gnielinski (cited by Schlilnder, 1978) of coefficient of heat transfer between particle and fluid. The wall coefficient may be taken as hw = 0.8hp.

An intermediate product with a melting point of 50 °C should be stored during two months in a cylindrical tank at 60 °C. The tank is a cylinder with vertical axis equipped with a jacket on the vertical wall allowing hot water circulation, but no agitator. The bottom and the lid are not heated. The volume is 4 m3, the height 1.8 m, and the diameter 1.2 m. The corresponding shape factor is 8( ri, = 2.37. The overall heat transfer coefficient of the jacket is 50Wrn 2 K 1. 

The area for heat transfer varies with radius therefore, we define the overall heat transfer coefficient based upon the outer area of the cylinder, A0=2nr0L, where L is the length of the cylinder, giving... 

We again see that the reciprocal of the overall heat transfer coefficient is equivalent to the overall resistance to heat transfer, which is equal to the sum of resistances in the hot film (1 //zG), the wall [r0 n(r0/ri)]/kw and the cold film (r0 r- ), as shown in Equation (22). However in this case, the expressions for the resistances in the wall and the cold film are modified to take the curvature into account. No such modification is required for the outside film resistance, as the overall heat transfer coefficient is defined based on the outside surface area of the cylinder. 

The heat transfer coefficient, h, is the variable whose value is being sought. Consider a series of bodies of the same geometrical shape, e.g., a series of elliptical cylinders (see Fig. 1.13), but of different size placed in various fluids. It can be deduced, either by physical argument or by considering available experimental results, that if, in the case of gas flows, the velocity is low enough for compressibility effects to be ignored, h will depend on ... 

A short concrete cylinder 15 cm in diameter and 30 cm long is initially at 25°C. It is allowed to cool in an atmospheric environment in which the temperature is 0°C. Calculate the time required for the center temperature to reach 6°C if the heat-transfer coefficient is 17 W/m2 °C. 

A stainless steel cylinder (18% Cr, 8% Ni) is heated to a uniform temperature of 200°C and then allowed to cool in an environment where the air temperature is maintained constant at 30°C. The convection heat-transfer coefficient may be taken as 200 W/m2 C. The cylinder has a diameter of 10 cm and a length of 15 cm. Calculate the temperature of the geometric center of the cylinder after a time of 10 min. Also calculate the heat loss. 

Fand [21] has shown that the heat-transfer coefficients from liquids to cylinders in cross flow may be better represented by the relation... 

Giedt, W. H. Investigation of Variation of Point Unit-Heat-Transfer Coefficient around a Cylinder Normal to an Air Stream, Trans. ASME, vol. 71, pp. 375-381, 1949. 

Conduction with Heat Source Application of the law of conservation of energy to a one-dimensional solid, with the heat flux given by (5-1) and volumetric source term S (W/m3), results in the following equations for steady-state conduction in a flat plate of thickness 2R (b = 1), a cylinder of diameter 2R (b = 2), and a sphere of diameter 2R (b = 3). The parameter b is a measure of the curvature. The thermal conductivity is constant, and there is convection at the surface, with heat-transfer coefficient h and fluid temperature I. ... 

Forced convection heat transfer is probably the most common mode in the process industries. Forced flows may be internal or external. This subsection briefly introduces correlations for estimating heat-transfer coefficients for flows in tubes and ducts flows across plates, cylinders, and spheres flows through tube banks and packed beds heat transfer to nonevaporating falling films and rotating surfaces. Section 11 introduces several types of heat exchangers, design procedures, overall heat-transfer coefficients, and mean temperature differences. 

Fia. 4. Heat-transfer coefficient to wall. Broken line is Nh according to Thoenes and Kramers (Tl). Curves (I) Aerov and Umnik (A2) (2) Coberly and Marshall (C4) (3) Hanratty (cylinders) (HI) (4) Hanratty (spheres) (HI) (5) Quinton and Storrow (Ql) (6) Yagi and Wakao (Y2). 

To begin the identification of the parameters with the Gauss-Newton method, the mathematical model of the process must be available. This model allows computation of the values of the temperature at the centre and the surface of the cylinder. At the same time, to estimate the starting vector of parameters (Pq), the method needs a first evaluation of the thermal conductivity 2.q and of the heat transfer coefficient Uq. 

Here jh — ( ioo/Cph( ) (Cp(i/A )fand hioc is the local heat-transfer coefficient G is the superficial mass velocity the subscript f denotes properties evaluated at the film temperature Tt = i Ts + Ty,). Here Ts refers to the surface temperature and Th to the bulk fluid temperature. The quantity l/ is an empirical coefficient that depends on the particle shape, e.g., yp = 1.0 for spheres and yp = 0.91 for cylinders. Values of yp for other shapes are tabulated elsewhere (B2, B3). 

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