Convective heat flow

Radiation transfer is largely eliminated when an insulant is placed in close contact with a hot surface. Radiation may penetrate an open-cell material but is rapidly absorbed within the immediate matrix and the energy changed to conductive or convective heat flow. It is also inhibited by the use of bright aluminum foil, either in the form of multi-corrugated sheets or as outer facing on conventional insulants. 

With so many variables, expressions for convective heat flow cannot be as simple as those for conduction. The interpretation of observed data has been made possible by the use of a number of groups which combine the variables and which can then be used to estimate convective heat flow. 

In the nonisothermal case, the plug flow model accounts only for the convective heat flow as shown in Figure 5.3. 

Subscript a refers to the air portion, subscripts a + 1 and a - l refer to the upper and lower air portion, index k refers to the four cell slices. Qcv,a is the convective heat flow exchanged with the tube and with each cell slice, which can be calculated ... 

Another area of research that could be profitably explored is the use of remote sensing instruments to measure surface temperatures of textile assemblies. Infrared thermovision cameras have been used to visualize temperature distributions over clothed and nude persons in order to study the transport of microorganisms by convective heat flow (112). A variety of less expensive radiometers and radiation pyrometers that are used to measure and automatically control the temperature of textiles during drying and texturing (113, llU, 115) could also assess the thermal behavior of apparel and clothing assemblies and thus elucidate their contribution to thermal comfort indoors. 

When there is a convective heat flow effect, we may define a combined energy flow e by... 

This indicates that one part of the heat does not flow directly from wall to fluid. A longitudinal heat flow exists and Agostini [30] and Commenge [31] give a rule to estimate whether or not the conditions required for a purely transversal heat flow are fulfilled. They define a Biot number which allows us to compare the convective heat flow and the conductive longitudinal heat flow. The former gives the definition... 

To maximize heat flow (Qc), the preheater must maximize hc and AT. The convective heat flow coefficient (//,.) is a function of the combustion gas velocity, which is a function of the burner firing rate and design. To maximize hc, a preheater should incorporate a high-fire/low-fire logic. This approach allows the burner to fire at high fire whenever the ladle requires energy so that maximum combustion gas velocity is maintained. This approach enables the preheater to achieve maximum heat flow at the most efficient times. 

Natural and forced convection. The forces used to create convection currents in fluids are of two types. If the currents are the result of buoyancy forces generated by differences in density and the differences in density are in turn caused by temperature gradients in the fluid mass, the action is called natural convection. The flow of air across a heated radiator is an example of natural convection. If the currents are set in motion by the action of a mechanical device such as a pump or agitator, the flow is independent of density gradients and is called forced convection. Heat flow to a fluid pumped through a heated pipe is an example of forced convection. The two kinds of force may be active simultaneously in the same fluid, and natural and forced convection then occur together. 

The considered radial process in the bentonite annulus is a complicated one with coupled, highly nonlinear flows that involve many things. There are liquid flow and vapor flow as well as conductive and convective heat flow depending on gradients in pressure, water vapor density and temperature. The flow coefficients depend on water properties such as saturation water vapor pressure and dynamic viscosity of water. They also depend on the properties of bentonite water retention curve, hydraulic conductivity and water vapor diffusion coefficient, and thermal conductivity, all of which are functions of degree of water saturation. 

Heat Input. Most thermal analyses use a heat source distributed over the tool surface, with a heat flux per unit area (in W/m ) of constant intensity or prescribed spatial variation (e.g., with radius from the tool axis). This recognizes that the heat is generated at the interface by friction or by viscous dissipation in a layer that is sufficienfly thin to consider it to be at the interface. Alternatively, for the probe, its share of the heat input may be distributed over the probe volume. In heat flow problems, such approximations rapidly lose significance as the distance from the heat source increases. The second assumption is usually to ignore tool tilt and the effect of metal flow on the distribution of heat generation and to treat the source as axisymmetric. The high rotational speed and consequent convective heat flow by motion of the material act to smooth out the circumferential distribution of heat. As an example, the heat input intensity has been represented as follows (Ref 17) ... 

As the battery exchanges heat with the environment, radiative and convective heat flows are apphed to the outermost discretization element. These two boundary conditions depend on... 

The method used to measure the wet bulb temperature is illustrated in Fig. 9.3-4, where a thermometer is covered by a wick or cloth. The wick is kept wet by water and is immersed in a flowing stream of air-water vapor having a temperature of T (dry bulb temperature) and humidity H. At steady state, water is evaporating to the gas stream. The wick and water are cooled to T,y and stay at this constant temperature. The latent heat of evaporation is exactly balanced by the convective heat flowing from the gas stream at T to the wick at a lower temperature T. ... 

In large extruders the convective heat flow generally dominates the conductive heat transfer to the wall (Gz 1). The process approaches an adiabatic situation, and all the heat produced by the reaction or by viscous dissipation results in a temperature rise. In this case the equations for temperature rise can be simplified even further. 

The heat conductivity of the solid material (polymer) vp is, in general, 10 to 100 times greater than Xg, the heat conductivity through the pores. Because of entrainment with water vapour molecules the convective heat flow through the membrane pores, is given... 

Convection, also known as Newtonian cooling (after Sir Isaac Newton), is a mechanism of heat transfer that occurs only in fluids. It involves the transfer of thermal energy by the mixing of fluids. The amount of convective heat transfer is a function of surface area in contact with the fluid, the temperature difference between the solid and the fluid, and the properties of the fluid. There are two types of convective heat flow — natural (or free) and forced. 

In convective heat flow, heat transfer takes effect by means of two mechanisms the direct conduction through the fluid, and the motion of the fluid itself Figure 2.2 illustrates convective heat transfer between a flat plate and a moving fluid. 

Radiation, thermal - The mode of heat flow that occurs by emission and absorption of eleetromagnetic radiation, propagating at the speed of light and, unlike conductive and convective heat flow, capable of propagating across a vacuum the form of heat transfer that allows infrared thermography to work since IR energy travels from the target to the detector by radiation. 

Convective Heat Flows into the Vapour and Liquid... 

Convective Heat Flows into the Vapour and Liquid 3.5.1 Convective Circulation in the Vapour... 

InaU cases, the induced recirculating downward mass flow has been discovered to carry heat down towards the liquid/vapour interface. In flie case of good insulation practice, this residual convective heat flow contributes to the boil-off, and may become the major source of heat entering the liquid. (The term residual is used to distinguish this heat flow from all other sources.)... 

Firstly, a series of convective Benard-type cells, with a vertical dimension of about 5 mm, carry superheated liquid up to within about 0.4 mm of the surface through a temperature gradient of the order of 50 K/m. i.e. the convective heat flow is transferred by a strong thermal process with a small impedance. 

Abstract The chapter concentrates on the sequence to be expected if stratification in LNG or LPG cannot be prevented. The natural convective heat flows into the stratified layers will lead to density equilibration between flie layers, when spontaneous mixing or roll-over may lead to a large rise in BOR. 

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