Convective heat exchange coefficient

Convective heat exchange coefficient A complicated factor involving the surface ge... 

A least-squares optimization routine is employed to systematically adapt the heat capacity, thermal conductivity, and convective heat exchange coefficient of the simulation model until a good agreement between measured and simulated impedance spectra is achieved. Final values of the parameter variation process represent the thermal parameters of the real battery. Figure 8 compares an impedance spectrum from measurement data with an impedance spectrum derived from the result values of the optimization process. As a good agreement... 

Correlations for Convective Heat Transfer. In the design or sizing of a heat exchanger, the heat-transfer coefficients on the inner and outer walls of the tube and the friction coefficient in the tube must be calculated. Summaries of the various correlations for convective heat-transfer coefficients for internal and external flows are given in Tables 3 and 4, respectively, in terms of the Nusselt number. In addition, the friction coefficient is given for the deterrnination of the pumping requirement. 

Effect of Uncertainties in Thermal Design Parameters. The parameters that are used ia the basic siting calculations of a heat exchanger iaclude heat-transfer coefficients tube dimensions, eg, tube diameter and wall thickness and physical properties, eg, thermal conductivity, density, viscosity, and specific heat. Nominal or mean values of these parameters are used ia the basic siting calculations. In reaUty, there are uncertainties ia these nominal values. For example, heat-transfer correlations from which one computes convective heat-transfer coefficients have data spreads around the mean values. Because heat-transfer tubes caimot be produced ia precise dimensions, tube wall thickness varies over a range of the mean value. In addition, the thermal conductivity of tube wall material cannot be measured exactiy, a dding to the uncertainty ia the design and performance calculations. 

The values of CJs are experimentally determined for all uncertain parameters. The larger the value of O, the larger the data spread, and the greater the level of uncertainty. This effect of data spread must be incorporated into the design of a heat exchanger. For example, consider the convective heat-transfer coefficient, where the probabiUty of the tme value of h falling below the mean value h is of concern. Or consider the effect of tube wall thickness, /, where a value of /greater than the mean value /is of concern. 

Although final heat-exchanger designs will be made on the basis of careful calculations of U, it is helpful to have a tabulation of values of the overall heat-transfer coefficient for various situations which may be encountered in practice. Comprehensive information of this sort is available in Refs. 5 and 6, and an abbreviated list of values of U is given in Table 10-1. We should remark that the value of U is governed in many cases by only one of the convection heat-transfer coefficients. In most practical problems the conduction resistance is small compared with the convection resistances. Then, if one value of h is markedly lower than the other value, it will tend to dominate the equation for U. Examples 10-1 and 10-2 illustrate this concept. 

The above derivation for LMTD involves two important assumptions (1) the fluid specific heats do not vary with temperature, and (2) the convection heat-transfer coefficients are constant throughout the heat exchanger. The second assumption is usually the more serious one because of entrance effects, fluid viscosity, and thermal-conductivity changes, etc. Numerical methods must normally be employed to correct for these effects. Section 10-8 describes one way of performing a variable-properties analysis. 

Lobing and dissection (Fig. 7-llb) tend to decrease the effective length across a leaf in the direction of the wind and hence to reduce Sbl (Eq. 7.10), with a consequent increase of convective heat exchange. For instance, the heat convection coefficient hc (Eq. 7.17) increases with the depth of leaf serrations. In addition to differences in size, the greater lobing observed for sun leaves compared to shade leaves on the same plant further reduces the heating of sun leaves above air temperature. Also, heat convection is greater for a pinnate leaf with many leaflets than for a simple undivided (entire) leaf of the same area (Fig. 7-llb). 

The inner and outer surfaces of a 25-cm-thick wall in summer are at 27"C and 44 C, respectively. The outer surface of the wall exchanges heal by radiation with surrounding surfaces at 40°C, and convection svith ambient air also at 40 C with a convection heat transfer coefficient of 8 W/m - °C. Solar radiation is incident on the surface at a rate of 150 W/m If both the emissivity and the solar absorptivity of the outer surface are 0.8, determine the effective thermal conductivity of the wall. 

A solar heat flux q, is incident on a sidewalk whose thermal conductivity is k, solar absorptivity is a and convective heat transfer coefficient is h. TaWng the positive x direction to be towards the sky and disregarding radiation exchange with the surroundings surfaces, the correct boundary condition for this sidewalk surface is... 

The use of fins is most effective in applications involving a low convection heat transfer coefficient. Thus, the use of fins is more easily justified when the medium is a gas instead of a liquid and the heat transfer is by natural convection instead of by forced convection. Therefore, it is no coincidence that in liquid-to-gas heat exchangers such as the car radiator, fins are placed on the gas side. 

A double pipe (shell-and-tube) heat exchanger is constructed of a stainless steel [k = 15.1 W/m O inner lube of inner diameter O/ = 1.5 cm and outer diameter 1.9 cm and an outer shell of inner diameter 3,2 cm. The convection heat transfer coefficient is given to be h,- = 800 W/m °C on the inner surface of the tube and h = 1200 W/m °C on the outer surface. For a fouling factor of f f, - 0.0004 m °C/W on the tube side and Ri =- 0.0001 m °C/W on the shell side, determine (a) the thermal resistance of the heat exchanger per unit iength,and (6) the overall heat transfer coefficients, Ujand U based on the inner and puter surface areas 0) the tube, respectively. 

Heat exchangers are complicated devices, and the results obtained with the simplifled approaches presented above should be used with care. For example, we assumed that the overall heat transfer coefficient V is constant throughout the heat exchanger and tliat the convection heat transfer coefficients can he predicted using the convection correlations. However, it should be kept in mind that the uncertainty in the predicted value of U can exceed 30 percent. Thus, it is natural to tend to overdesign the hear exchangers in order to avoid unpleasant surprises. 

The convective heat transfer coefficients at the surfaces immersed in the bed are high. This property indicates that internal heat exchangers require relatively small surface areas. 

The heat transfer between the solid particles and the fluid follows the local thermal non-equilibrium model. The volumetric heat transfer coefficient hv represents the heat exchanger capacity of the porous media. In the current paper, we will investigate the volumetric convection heat transfer coefficient by numerical simulation results of the... 

Coefficient of convective heat exchange [ W/m K] Ratio of vapor chemical potential to that of liquid [1]... 

In double-pipe and shell-and-tube heat exchangers, fluids flow through straight, smooth pipes and tubes of circular cross section. Many correlations have been published for the prediction of the inside-wall, convective heat transfer coefficient, /i when no phase change occurs. For turbulent flow, with Reynolds numbers, = D,G/ ji, greater than 10,000, three empirical correlations have been widely quoted and applied. The first is the Dittus-Boelter equation (Dittus and Boelter, 1930) for liquids and gases in fully developed flow (Z>,/L < 60), and with Prandtl numbers, = Cp[iJk, between 0.7 and 100 ... 

Correlations are available for predicting pressiffe drops and convective heat transfer coefficients for laminar flow inside and outside of ducts, tubes, and pipes for pipes with longitudinal and peripheral fins for condensation and boiling and for several different geometries used in compact heat exchangers. No attempt is made to discuss or summarize these correlations here. They are presented by Hewitt (1992). 

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