Convective heat transfer process

More recently, Zucrow et al. (Zl) have run experiments which show that in the region of low flow rates the burning rates of certain propellants actually decrease with increasing gas flow. As the gas flow rate increases, the burning rate is observed to go through a minimum and then increase with further increases in gas flow rate. The decrease in burning rate was attributed to undefined mass-transfer processes. Eventually, the convective heat-transfer processes overcome this effect to give results similar to those obtained by others. 

Micfov/ave ovens replace the slow convection heat transfer process in conventional ovens by the instantaneous radiation heat transfer. As a result, micro-wave ovens transfer energy to the food at full capacity the moment they are turned on, and thus they cook faster while consuming less energy. 

The evolution of the local temperature in a reacting system is governed by the heat balance equation. This may become quite complex in unstirred systems, especially if convective heat transfer processes develop as a consequence of local heating. For the simple CSTR described earlier we can proceed with an ordinary differential equation of the form... 

Sustained combustion requires a continuous supply of fresh reactants and a continuous removal of reaction products. This process is loosely known as mass transfer. Specifically, mass transfer is a consequence of three possible modes bulk fluid motion, molecular and turbulent diffusion, and reaction sources and sinks. Mass transfer due to bulk fluid motion is generally known as convection. It is similar to the convection heat transfer process. Mathematically, the rate of change for species / per unit volume, pYit via convection can be described as 3(pUjY ldxj, where p is fluid density, Yt is the mass fraction of species i, Uj is the / -component of the fluid velocity. 

The results of investigation of similar convective heat transfer processes can be used in the design and analysis of heat exchangers. 

This convective heat transfer process follows the treatment of the gas gap, in that the descriptive equation is ... 

Computer modeling of convection has had mixed success. Many convection problems, particularly those involving laminar flow, can readily be solved by special computer programs. However, in situations where turbulence and complex geometries are involved, computer analysis and modeling are still under development. Mass transfer analogies can play a key role in the study of convective heat transfer processes. Two mass transfer systems, the sublimation technique and the electrochemical technique, are of particular interest because of their convenience and advantages relative to direct heat transfer measurements. 

The liquid and solid cooling stages, that is, stage one and stage three, are described by a convective heat transfer process. Since the heat exchange due to mass transfer and radiation is neglected, these two stages can be described by means of (16.1)-(16.3), with qm = qr = 0. 

DHT is a convective heat transfer process that is strongly influenced by the flow characteristics... 

Certainly, the most important convective heat-transfer process industrially is that of cooling or heating a fluid flowing inside a closed circular conduit or pipe. Diflferent types of correlations for the convective coefficient are needed for laminar flow below 2100), for fully turbulent flow above 6000), and for the transition region (/Vr between 2100 and 6000). 

Like in the convection heat transfer process, the convective mass transfer of species i from the moving bulk fluid to the solid surface is given by a convection rate equation analogous to Newtcm s law of cooling as... 

Solution Polymerization. In this process an inert solvent is added to the reaction mass. The solvent adds its heat capacity and reduces the viscosity, faciUtating convective heat transfer. The solvent can also be refluxed to remove heat. On the other hand, the solvent wastes reactor space and reduces both rate and molecular weight as compared to bulk polymerisation. Additional technology is needed to separate the polymer product and to recover and store the solvent. Both batch and continuous processes are used. 

The maximum velocity at the axis is twice the average, whereas the velocity at the wall is zero. The effect of the burner wall is to cool the flame locally and decrease the burning velocity of the mixture. This results in flame stabilization. However, if the heat-transfer processes (conduction, convection, and radiation) involved in cooling the flame are somehow impeded, the rate of heat loss is decreased and the local reduction in burning velocity may no longer take place. This could result in upstream propagation of the flame. 

A discussion of retention time in rotary Idlns is given in Brit. Chem. Eng., 27-29 (Januaiy 1966). Rotary-ldln heat control is discussed in detail by Bauer [Chem. Eng., 193-200 (May 1954)] and Zubrzycki [Chem. Can., 33-37 (Februaiy 1957)]. Reduction of iron ore in rotaiy Idlns is described by Stewart [Min. Congr J., 34—38 (December 1958)]. The use of balls to improve solids flow is discussed in [Chem. Eng., 120-222 (March 1956)]. Brisbane examined problems of shell deformation [ Min. Eng., 210-212 (Februaiy 1956)]. Instrumentation is discussed by Dixon [Ind. Eng. Chem. Process Des. Dev., 1436-1441 (July 1954)], and a mathematical simulation of a rotaiy Idln was developed by Sass [Ind. Eng. Chem. Process Des. Dev., 532-535 (October 1967)]. This last paper employed the empirical convection heat-transfer coefficient given previously, and its use is discussed in later correspondence [ibid., 318-319 (April 1968)]. 

It is important to emphasize that, especially in process measurements, radiation can have an essential influence on the wet bulb temperature, and therefore generally the wet bulb temperature is dependent on the mea,surement device and the method of measurement. If the airflow is very low, the radiation can have a remarkable contribution in addition to the convective heat transfer. Basically, an equation analogous to Eq. (4.138) can be empirically determined for each wet bulb temperature and method of measurement. 

It is discovered that in the cooling tower the water moving downward from the jets changes its direction to upward after drop formation. There is an effective heat transfer process when the drops move upward heat transfers from the outlet air to the drops through convection and condensation. 

Most heat transfer processes used in production facilities involve combinations of conduction and convection ti ansfer processes. For example, in heat exchangers the transfer of heat energy from the hot fluid to the coLl fluid involves tliree steps. First, the heat energy is transferred from the luH fluid to the exchanger tube, then through the exchanger tube wall, ctud finally from the tube wall to the cold fluid. The first and third steps are convection transfer processes, while the second step is conduction process. 

A fire tube contains a flame burning inside a piece of pipe which is in turn surrounded by the process fluid. In this situation, there is radiant and convective heat transfer from the flame to the inside surface of the fire tube, conductive heat transfer through the wall thickness of the tube, and convective heat transfer from the outside surface of that tube to the oil being treated. It would be difficult in such a simation to solve for the heat transfer in terms of an overall heat transfer coefficient. Rather, what is most often done is to size the fire tube by using a heat flux rate. The heat flux rate represents the amount of heat that can be transferred from the fire tube to the process per unit area of outside surface of the fire tube. Common heat flux rates are given in Table 2-11. 

Where heat transfer is taking place at the saturation temperature of a fluid, evaporation or condensation (mass transfer) will occur at the interface, depending on the direction of heat flow. In such cases, the convective heat transfer of the fluid is accompanied by conduction at the surface to or from a thin layer in the liquid state. Since the latent heat and density of fluids are much greater than the sensible heat and density of the vapour, the rates of heat transfer are considerably higher. The process can be improved by shaping the heat exchanger face (where this is a solid) to improve the drainage of condensate or the escape of bubbles of vapour. The total heat transfer will be the sum of the two components. 

A mechanism of heat transfer in which heat energy is transmitted by convection current motion through gases and liquids. Part of the heat-transfer process in a boiler is by convection whereby the circulation of water carries heat from the tube near the fire to the drum and surrounding areas. 

In a number of these processes, a liquid heat- and mass-transfer medium is in direct contact with the catalyst and the reaction mixture. The main function of the liquid is to act as a heat sink and as a medium for convective heat transfer. However, since the liquid may be assumed to cover the solid particles and in this way act as a barrier between the gaseous and the solid phases, it must therefore also function as a mass-transfer medium. 

Convection. Heat transfer by convection arises from the mixing of elements of fluid. If this mixing occurs as a result of density differences as, for example, when a pool of liquid is heated from below, the process is known as natural convection. If the mixing results from eddy movement in the fluid, for example when a fluid flows through a pipe heated on the outside, it is called forced convection. It is important to note that convection requires mixing of fluid elements, and is not governed by temperature difference alone as is the case in conduction and radiation. 

Now that the overall coefficient U has been broken down into its component parts, each of the individual coefficients /q, hi, and hi must be evaluated. This can be done from a knowledge of the nature of the heat transfer process in each of the media. A study will therefore be made of how these individual coefficients can be calculated for conduction, convection, and radiation. 

Analytical solutions of equation 9.44 in the form of infinite series are available for some simple regular shapes of particles, such as rectangular slabs, long cylinders and spheres, for conditions where there is heat transfer by conduction or convection to or from the surrounding fluid. These solutions tend to be quite complex, even for simple shapes. The heat transfer process may be characterised by the value of the Biot number Bi where ... 

Heat transfer by convection occurs as a result of the movement of fluid on a macroscopic scale in the form of eddies or circulating currents. If the currents arise from the heat transfer process itself, natural convection occurs, such as in the heating of a vessel containing liquid by means of a heat source situated beneath it. The liquid at the bottom of the vessel becomes heated and expands and rises because its density has become less than that of the remaining liquid. Cold liquid of higher density takes its place and a circulating current is thus set up. 

An alternate to the concept of cluster renewal discussed above is the concept of two-phase convection. This second approach disregards the separate behavior of lean and dense phases, instead models the time average heat transfer process as if it were convective from a pseudo-homogeneous particle-gas medium. Thus h hcl, hh and hd are not... 

For the analysis, a steady-state fire was assumed. A series of equations was thus used to calculate various temperatures and/or heat release rates per unit surface, based on assigned input values. This series of equations involves four convective heat transfer and two conductive heat transfer processes. These are ... 

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