Heat transfer increase

As the heat transfer increases, the flame velocity decreases, but before the decrease becomes significant propagation stops. Quantitative verification is hampered by complicating factors convective motion of the gas, which bends the flame front, causes a difference between the theoretical rate of flame motion with respect to the gas and the measured rate of flame motion with respect to the walls. 

In this thin-film machine, the small clearance between heated wall and rotor blade, together with the high peripheral blade velocity, results in high shear gradients, whereby the apparent viscosity in the film is considerably reduced. The resulting increased turbulence and better surface renewal improve heat transfer, increase reaction velocities, and aid the required forced product flow on the wall. On the basis of test... 

We need no Nusselt number relations for the case of the hotter plate being at the top, since there are uo convection cuiients in this case and heat transfer is downward by conduction (Nu = 1). When the hotter place i.s at the bottom, however, significant convection currents set in for Ra > 1708, and the rate of heat transfer increases (Fig. 9-24). 

Moisture content also affects the effective conductivity of porous mediums such as soils, building materials, and insulations, aud thus heat transfer through them. Several studies have indicated that heat transfer increases almost linearly with moisture content, at a rate of 3 to 5 percent for each percent increase in moisture content by volume. In.sulation with 5 percent moisture content by volume, for example, increases heat transfer by 15 to 25 percent relative to dry insulation (ASHRAE Handbook of Fundamentals,... 

The particle size does not affect only the steam mass flow but also the heat transfer between the heat exchanger and the fluidised bed. The heat transfer against particle size shows a minimum between of 1.5 - 2.0 mm in this case. At smaller particle sizes the heat transfer increases quickly. 

The glass melting process always involves a fairly intensive melt flow it must be taken into account when melting in pots or tanks, but is of special significance in the case of continuous tank furnaces where it is a prerequisite of correct furnace function. Melt flow has the positive effect of accelerating the mass and heat transfer. Increased corrosion of refractories and possible carry-over of unmelted batch into the refining and working zone are its undesirable consequences. 

It should be stressed that the above-described approach is strictly valid at temperatures above 900°C, when the viscous flow represents the prevailing restraints to mass loss. This conclusion is reasonable, as the efficiency of heat transfer increases rapidly with an increasing temperature. 

Safe operation is a paramount concern in chemical reactor operations. Runaway reactions occur when the heat generated by the chemical reactions exceed the heat that can be removed from the reactor. The surplus heat increases the temperature of the reacting fluid, causing the reaction rates to increase further (heat generation increases exponentially with temperature while the rate of heat transfer increases linearly). Runaway reactions lead to rapid rise in the temperature and pressure. 

Kenning and Kao [226] noted heat transfer increases of up to 50 percent when nitrogen bubbles were injected into turbulent water flow. A similar level of enhancement was observed by Baker [227], who created slug flow in small rectangular channels with simulated microelectronic chips on one of the wide sides. 

Wall-to-Bed Heat Transfer. The wall-to-bed heat transfer coefficient increases with an increase in liquid flow rate, or equivalently, bed voidage. This behavior is due to the reduction in the limiting boundary layer thickness that controls the heat transport as the liquid velocity increases. Patel and Simpson [94] studied the dependence of heat transfer coefficient on particle size and bed voidage for particulate and aggregative fluidized beds. They found that the heat transfer increased with increasing particle size, confirming that particle convection was relatively unimportant and eddy convection was the principal mechanism of heat transfer. They observed characteristic maxima in heat transfer coefficients at voidages near 0.7 for both the systems. 

In conclusion, it is evident from Figs. 22 and 23 that the rate of heat-transfer increases with increasing interfacial turbulence, irrespective of whether this turbulence is induced by high heat fluxes (as in Gordon s mercury surface), induced boiling by artificial nucleation sites (gas or solid contaminants), or mechanical agitation at the liquid-liquid interface. 

Capacity increases in direct proportion to the area exposed per unit weight and in proportion to the heat transfer coefficient, which increases with average gas temperature and gas blanket thickness (figs. 2.13 and 2.14). Obviously, heat transfer increases as zone temperature setpoints are raised, unless scale formation interferes—as it will do if the preheat or entry zone is raised above 2300 F (1260 C). 

In Fig. 4.3-1 the Nusselt number is depicted as function of Re Pr d/L respectively Re Sc d/L. The highest transfer coefficients are attained for frictionless fluids (v = 0 or Pr = 5 c = 0). In case of developed laminar flow the transfer coefficients a and p only depend on the pipe diameter and the material properties. If the flow is not developed heat transfer increases with the flow velocity by d/L. ... 

To determine the heat transfer area of a heat exchanger from Eq. (13.7), an overall heat trans fer coefficient is required. It can be estimated from experience or from the sum of the indi vidual thermal resistances. For double-pipe and shell-and-tube heat exchangers, the area fo heat transfer increases across the pipe or tube wall from the inside to the outside surface. Ac cordingly, the overall heat transfer coefficient is based on the inner wall, i, the outer wall, o or, much less frequently, a mean, m. The three coefficients are related by... 

Equation 10 is the surface heat flux per unit area. Considering laminar flow and the decreasing thermal boundary layer with increasing velocity, the heat transfer increases in relation with the flow velocity. 

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