Nusselt number, heat exchanger

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. 

Heat Exchangers Since most cryogens, with the exception of helium 11 behave as classical fluids, weU-estabhshed principles of mechanics and thermodynamics at ambient temperature also apply for ctyogens. Thus, similar conventional heat transfer correlations have been formulated for simple low-temperature heat exchangers. These correlations are described in terms of well-known dimensionless quantities such as the Nusselt, Reynolds, Prandtl, and Grashof numbers. 

The minimum value of the Nusselt Number for which equation 9.216 applies is 3.5. Reynolds Numbers in the range 2000-10,000 should be avoided in designing heat exchangers as the flow is then unstable and coefficients cannot be predicted with any degree of accuracy. If this cannot be avoided, the lesser of the values predicted by Equations 9.214 and 9.216 should be used. 

Heat exchange in stirred reactors is described in [207]. By using dimensional analysis of heat flow and energy balance equations, the Nusselt number, containing hT, can be expressed as a function of the Reynolds number and the Prandtl number ... 

Nusselt number - [FLUIDIZATION] (Vol 11) - [HEAT-EXCHANGE TECHNOLOGY- HEAT TRANSFER] (Vol 12) - [ELECTROCHEMICALPROCESSDTG - INTRODUCTION] (Vol9) - [MIXING AND BLENDING] (Vol 16)... 

As the diameter is decreased, the heat transfer from a unit volume intensifies because of the increase in the ratio of surface to volume (d-1) heat exchange per unit surface also intensifies. For a constant value of the Nusselt number the heat exchange coefficient is proportional to d l. Under the rough assumption that Tc and E change little from one case to the next, we come to the conclusion that at the limit the Peclet number (numerically equal for gases to the Reynolds number), based on the flame velocity (or adiabatic flame velocity u0) has a specific value... 

In the Nusselt number, the term (q/AT), the rate of heat transfer per unit area of heat exchanger per unit temperature driving force, is known as the heat transfer coefficient and is given the symbol h. The heat transfer coefficient is used to characterise heat transfer rates. Heat transfer processes are described in more detail in Chapter 4, 11 An Introduction to Heat Transfer . 

In a heat exchanger, air flows through a pipe of length, l. The air enters the pipe at a temperature, Tt. The heat flux at the wall of this pipe increases linearly from zero at the inlet of the pipe to a value of qw at the end of the pipe. The velocity in the pipe is such that HRe PrD — 0.07 where D is the diameter of the pipe and Re is the Reynolds number. Determine how the Nusselt number based on the local wall heat transfer rate and on the difference between the local wall temperature and the inlet temperature varies with the dimensionless distance. Z, along the pipe. 

As was mentioned in Section I, heat transport phenomena of longitudinal flow through rod bundles (i.e., transport from the rods to the fluid v.v.) have been studied quite extensively in the past and are still receiving constant attention, due to the importance of compact heat exchangers and nuclear reactors. This has led to many theoretical and empirical relations to predict Nusselt numbers as a function of the relative pitch and (for nonlaminar flow) the Reynolds number a relatively recent review of those relations that pertain to smooth rod bundles is by Rehme [6]. 

The heat-transfer coefficient, k, for laminar flow in tubes can be enhanced by a factor of two- to sixfold using static mixer elements in the heat-exchanger tubes. The heat-transfer coefficient is correlated using the Nusselt, Prandtl, and Reynolds numbers. Table 9.21 gives constants for Koch SMX and SMXL mixers for... 

Heat exchange in fully developed laminar flow of fluids in tubes of various cross-sections was studied in many papers (e.g., see [80, 253, 341]). In what follows, we present some definitive results for the limit Nusselt numbers corresponding to the region of heat stabilization in the flow in the case of high Peclet numbers (when the molecular heat transfer can be neglected). 

For smooth particles of an arbitrary shape in ideal fluid (this model is used, say, to describe heat exchange between particles and liquid metals at Pr -C 1 and Re 3> 1) and in the absence of regions with closed streamlines, the mean Nusselt number can be calculated by the formula... 

Rectangular ducts are also often used in the design of heat transfer devices such as compact heat exchangers. Unlike circular and parallel plate ducts, two-dimensional analysis is required to obtain the friction factors and Nusselt numbers for rectangular ducts. 

Internally finned tubes are ducts with internal longitudinal fins. These tubes are widely used in compact heat exchangers. The friction factor-Reynolds number product and the Nusselt number for such internally finned tubes, designated as (/ Re), and Nu/,c> respectively, are computed from the following definitions ... 

The overall heat transfer coefficient consists of different contributions from heat transfer resistances, For simplicity, we do not consider different exchange areas of the various heat transfer resistances. The heat transfer coefficient in the reaction channel is correlated with the Nusselt number, whUe the thermal resistances in the wall and the cooling channel are lumped together and replaced with a coefficient [17]. 

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