Heat transfer free convective

Heat produced by viscous heating Conduction heat transfer Free convection heat transfer... 

Factors involved in heat transfer, such as surface-to-volume ratio, agitation characteristics, mixing efficiency, fouling of heat transfer surfaces, scale of operations, and the resulting heat exchanged depend on the system under consideration (e.g., liquid-liquid transfer, liquid-gas transfer, free convection, or forced convection). Standard chemical engineering texts and reference books contain detailed discussions on heat transfer in process equipment. Only a brief summary follows ... 

Numerical simulations of the thermal performance of the module were performed using finite element analysis. In the present model, the fluid path is represented by a series of interconnected nodes. Convection processes are modeled as transfer processes between these nodes (or volumes) and surfaces of the geometrical mesh. In this case, a series of analyses based on knowledge of the fluid properties, flow rates, and the relative sizes of the fluid passages and solid phase interconnections led to the value of 3.88 W/cm -K for the effective heat-transfer coefficient. Convective heat transfer using this coefficient was used on all of the internal free surfaces of the module. 

For the case of dense-gas heat- and mass transfer, free convective effects are dominant under certain conditions. In engineering work, use is made of these properties. 

In the region of slip flow, heat transfer by convection is significant only at a distance from the heat source greater than the mean free path. Within a distance of one mean free path, just as in molecular flow, heat transfer occurs by molecular conduction alone. Kyte et al., describe a method of calculation for heat transfer in slip flow (K2). The technique employed is essentially a trial and error solution whereby a temperature, < , is determined at a distance of one mean free path away from the heat source. The rate of heat flow from the hot surface is calculated on the basis of molecular conduction over the distance of one mean free path. The rate of heat flow beyond the distance of one mean free path is calculated on the basis of convection. The correct value of the temperature is that which results in equal rates of heat flow by conduction and by convection. 

Effect of free-molecular conduction important (a) for heat transfer by convection,... 

A. Edwards and B. N. Furber, The Influence of Free-Stream Turbulence on Heat Transfer by Convection From an Isolated Region of a Plane Surface in Parallel Air Flow, Proc. Inst. Mech. Eng. (170) 941,1956. 

Heat transfer by convection occurs in liquids and gases where there is a velocity field caused by extorted fluid motion or by natural fluid motion caused by a difference in density. The former case involves forced convection, and the latter case free convection. Combined convection occurs when both forced and free convection are present. The convection coefficient of surface heat transfer, a, defining the heat exchange in the contact boundary layer between fluid and soUd, is determined. Coefficient or is often expressed by equations containing criteria numbers, such as those of Nusselt (Nu), Prandtl (Pr), Reynolds (Re) and Grashof(Gr) ... 

The energy balance, see (20.40), is the sum of the energy becoming free during the reaction and the heat transfer through the waU of the inner pipe. Then the quotient of the dimensionless temperature 9, which is related to the initial drop temperature, is also dependent on the conversion and the heat transfer, calculated by an overall heat transfer coefficient Uw This heat transfer coefficient takes into account the heat transfer by conduction through the walls and the heat transfer by convections, which is calculated with the use of the Nusselt number. The specific enthalpy of reaction is included in ATad, representing the adiabatic temperature. 

Rayleigh number, Ra p L Cp AT k fl coefficient of thermal volume expansion AT temperature different across film L characteristic length dimension heat transfer by convection heat transfer by conduction Free convection... 

Assume that in six months the pressure in the vacuum space of a liquid-nitrogen line will rise to iM- From Fig. 2 the pressure of the line while in use would be 5 10 mm Hg, and the heat transfer to the bare liquid line, from Fig. 5, would be 2.2 10 w/cm. The AT required between the outer line and ambient air to transfer this much heat by free convection would be S F. 

Convective heat transfer is classified as forced convection and natural (or free) convection. The former results from the forced flow of fluid caused by an external means such as a pump, fan, blower, agitator, mixer, etc. In the natural convection, flow is caused by density difference resulting from a temperature gradient within the fluid. An example of the principle of natural convection is illustrated by a heated vertical plate in quiescent air. 

As an example, for free convective heat transfer from a vertical wall. 

In this section the correlations used to determine the heat and mass transfer rates are presented. The convection process may be either free or forced convection. In free convection fluid motion is created by buoyancy forces within the fluid. In most industrial processes, forced convection is necessary in order to achieve the most economic heat exchange. The heat transfer correlations for forced convection in external and internal flows are given in Tables 4.8 and 4.9, respectively, for different conditions and geometries. 

The convective heat transfer for the panel is free convection from a heated surface faced down. It can be calculated from Incropera and DeWitt ... 

Convection is the heat transfer in the fluid from or to a surface (Fig. 11.28) or within the fluid itself. Convective heat transport from a solid is combined with a conductive heat transfer in the solid itself. We distinguish between free and forced convection. If the fluid flow is generated internally by density differences (buoyancy forces), the heat transfer is termed free convection. Typical examples are the cold down-draft along a cold wall or the thermal plume upward along a warm vertical surface. Forced convection takes place when fluid movement is produced by applied pressure differences due to external means such as a pump. A typical example is the flow in a duct or a pipe. 

The solar radiation absorbed on external building surfaces increases the wall surface temperature, thus leading to a change in the heat conducted through the component. In low-wind conditions, free convective flows drift up the warm external wall surface. This changes the convective heat transfer and leads to increased temperatures of supply air for natural ventilation. 

Convection is heat transfer between portions of a fluid existing under a thermal gradient. The rate of convection heat transfer is often slow for natural or free convection to rapid for forced convection when artificial means are used to mix or agitate the fluid. The basic equation for designing heat exchangers is... 

This design is not well adapted to free-convection heat transfer outside a tube or coil therefore, for this discussion only agitation is considered using a submerged helical coil, Oldshue and Kern . 

Wiebelt,J. A.,J. B. Henderson, andj. D. Parker, Free Convection Heat Transfer from the Outside of Radial Fin Tubes, Heat Trans. Eng.,Y. 4, April une (1980) p. 53. 

Convection requires a fluid, either liquid or gaseous, which is free to move between the hot and cold bodies. This mode of heat transfer is very complex and depends firstly on whether the flow of fluid is natural , i.e. caused by thermal currents set up in the fluid as it expands, or forced by fans or pumps. Other parameters are the density, specific heat capacity and viscosity of the fluid and the shape of the interacting surface. 

FI. Farber, E. A., Free convection heat transfer from electrically heated wires, J. Appl. Phys. 22, 1437 (1951). 

In most cases where convective heat transfer is taking place from a surface to a fluid, the circulating currents die out in the immediate vicinity of the surface and a film of fluid, free of turbulence, covers the surface. In this film, heat transfer is by thermal conduction and, as the thermal conductivity of most fluids is low, the main resistance to transfer lies there, Thus an increase in the velocity of the fluid over the surface gives rise to improved heat transfer mainly because the thickness of the film is reduced. As a guide, the film coefficient increases as (fluid velocity)", where 0.6 < n < 0.8, depending upon the geometry. 

Kato H NtSHtWAKi, N. and Hirata, M. Im. Jl. Heat Mass Transfer 11 (1968) 1117. On the turbulent heat transfer by free convection from a vertical plate. 

Heat transfer coefficient a (for free convection) for heating from a wall a = 3-6 W/m2 K (surface vertical), for floor heating a = 6-10 W/m2 K (surface horizontal) and no heating from ceilings, as the temperature gradient would suppress convection ... 

Cheng, L. Y., andP. R.Tichler, 1991, CHF for Free Convection Boiling in Their Rectangular Channels, ANS Proc. Natl. Heat Transfer Conf., Minneapolis, MN, pp. 83-90. (2)... 

Kutateladze, S. S., V. N. Moskvicheva, G. I. Bobrovich, N. N. Mamontova, and B. P. Avksentyuk, 1973, Some Peculiarities of Heat Transfer Crisis in Alkali Metals Boiling under Free Convection, Int. 

Another case of free convection with some complications, but amenable to solution, is that due to combined temperature and concentration differences. De Leeuw den Bouter et al. (DIO) experimented with such combined free convective transfer, assuming complete analogy of heat and mass transfer if the Grashof number employed is of the form... 

Finally, there are some miscellaneous polymer-electrolyte fuel cell models that should be mentioned. The models of Okada and co-workers - have examined how impurities in the water affect fuel-cell performance. They have focused mainly on ionic species such as chlorine and sodium and show that even a small concentration, especially next to the membrane at the cathode, impacts the overall fuelcell performance significantly. There are also some models that examine having free convection for gas transfer into the fuel cell. These models are also for very miniaturized fuel cells, so that free convection can provide enough oxygen. The models are basically the same as the ones above, but because the cell area is much smaller, the results and effects can be different. For example, free convection is used for both heat transfer and mass transfer, and the small... 

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