Heat transfer coefficient variables influencing

To carry out the experiments in a meaningful and systematic way, it will be necessary, first, to consider one of the parameters as a variable while keeping the others constant and then to measure the corresponding pressure drop. The same type of experiment is carried out for the measurement of the heat transfer coefficient. Contrary to the mass transport pressure drop, which could be measured directly, the heat transfer coefficient is obtained indirectly by measuring the temperature of the wall and of the fluid at the entrance and exit of the pipe. The determination of the functional relationship between Ap/1, a and the various parameters that influence the process is illustrated in Fig. 6.1. 

The convection heat transfer coefficient /i is not a property of the fluid. It is an experimentally determined parameter whose value depends on all the variables influencing convection such as the surface geometry, the nature of fluid motion, Ihe properties of the fluid, and the bulk fluid velocity. Typical values of h arc given in Table 1-5. 

Finally, it should also be pointed out that in heat conduction problems the dimensionless representation and the combination of the influencing quantities into dimensionless numbers are not as significant as in the representation and determination of heat transfer coefficients in 1.1.4. In the following sections we will frequently refrain from making the heat conduction problem dimensionless and will only present the solution of a problem in a dimensionless form by a suitable combination of variables and influencing quantities. 

D. C. McMurray, P. S. Meyers, and O. A. Uyehara, Influence of Impinging Jet Variables on Local Heat Transfer Coefficients along a Flat Surface with Constant Heat Flux, in Proceedings of the Third International Heat Transfer Conference, 2, pp. 292-299, AIChE, New York, 1966. 

The reaction diagram for a single component reaction, as shown in Fig. 7.2 has two adjustable parameters, the amount of initiator and therefore the radical concentration and the temperature, all other parameters depend on changes in these two primary variables (2). Due to the deeomposition kinetics, the radical concentration depends of course also on the temperature of the reacting mixture. In reactive extrusion this temperature is affected by the wall temperature and the heat transfer coefficient but also by the reaction velocity, which, in turn, is influenced by the radical concentration and the temperature. The reaction velocity, together with the residence time determines the conversion, which has its effect on the viscosity of the reacting mixture. The viscosity is also affected by the average... 

The radical concentration is a primary variable of the reaction. Nevertheless, it can be influenced by the temperature of the reactive mixture, which is influenced by the wall temperature. Besides the wall temperature, the mixing temperature is also affected by the reaction velocity and the heat-transfer coefficient, which, in its turn, is influenced by the screw speed and throughput. 

The heat and mass transfer fluxes at the interface are influenced by the fluid d3mamics of the phases near the interface. In most situations the flow in this region is so complicated that the governing equations cannot be solved without introducing simplifying assumptions. The fluid d3mamic model adopted should, however, still be able to capture the main characteristics of the actual flow, as the heat and mass transfer coefficients may be very sensitive upon these variables. 

In most studies dealing with heat and mass transfer, it has been generally assumed that the thermo-physical properties, such as thermal conductivity, specific heat, molecular diffusivity of non-Newtonian polymer solutions, are the same as that for water, except for their non-Newtonian viscosity. Intuitively, one would expect the surface tension to be an important variable by way of influence on bubble dynamics and shape, but only a few investigators have controlled/measured/included it in their results. The available correlations can be broadly classified into two types first, those which directly relate the volumetric mass transfer coefficient with the liquid viscosity and gas velocity. The works of Deckwer et al. [36], Godbole et al. [42] and Ballica and Ryu [60] illustrate the applicability of this approach. All of them have correlated their results in the following form ... 

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