Heat transfer fluid velocity

Air (heat transfer fluid) velocity is set at a value recommended by the oven manufacturer. This value is based on experimentation to maximize the heat transfer (flow is fairly turbulent) to the mold. 

Figure 10-50C. Tube-side (inside tubes) liquid film heat transfer coefficient for Dowtherm . A fluid inside pipes/tubes, turbulent flow only. Note h= average film coefficient, Btu/hr-ft -°F d = inside tube diameter, in. G = mass velocity, Ib/sec/ft v = fluid velocity, ft/sec k = thermal conductivity, Btu/hr (ft )(°F/ft) n, = viscosity, lb/(hr)(ft) Cp = specific heat, Btu/(lb)(°F). (Used by permission Engineering Manual for Dowtherm Heat Transfer Fluids, 1991. The Dow Chemical Co.)...

Fig. 7. Simulation results of bubble formation and rising in Paratherm NF heat-transfer fluid with and without particles. Nozzle size 0.4cm I.D., liquid velocity Ocm/s, gas velocity lOcm/s, and particle density 0.896 g/cm3. (a) No particle (b) 2000 particles (c) 8000 particles (d) 8000 particles.

Velocity and temperature fields are therefore only similar when also the dimensionless groups or numbers concur. These numbers contain geometric quantities, the decisive temperature differences and velocities and also the properties of the heat transfer fluid. The number of dimensionless quantities is notably smaller than the total number of all the relevant physical quantities. The number of experiments is significantly reduced because only the functional relationship between the dimensionless numbers needs to be investigated. Primarily, the values of the dimensionless numbers are varied rather than the individual quantities which make up the dimensionless numbers. 

Pujo, M. and Bott, T.R., 1991, Effects of fluid velocities and Reynolds Numbers on biofilm development in water systems, in Keffer, J.F., Shah, R.K. and Ganic, E.N. eds. Experimental Heat Transfer, Fluid Mechanics and Thermodynamics. Elsevier, New York, 1358 - 1362. 

The Sieder and Tate relation involving coefficient of heat transfer, mass velocity, physical properties of a fluid and inside tube diameter is shown in Figure 2-25 in terms of dimensionless groups with Reynolds number as abscissa. It will be seen from Figure 2-25 that there are three distinct zones of flow. The first is the streamline region for values of Reynolds number of 2,100 and less. The series of parallel lines is expressed by the equation shown in Figure 2-25. 

Heat-transfer surfaces, velocities of fluids, coefficients. 

When the air-content is 10%, and the fluid inlet velocity is the only parametric conditions, the distance of the radial heat transfer in two-phase flow is increase and then decreases, so there is a peak value. The simulation results show when the inlet velocity is 50 m/s, there is a biggest heat transfer distance value, which is 1.8 m. The smallest one is 1.1 m under the speed of 5 m/s. The average temperature of the wall inlet velocity of two-phase fluid can be calculated through the scatter plot, and the outlet temperature of the outlets in this two-phase flow can be determined by this way, which is 21.21 °C. When the fluid inlet velocity is 5 m / s, the average outlet temperature of the two-phase fluid is 18.24 °C, higher than the inlet 3.24 °C, and it is the minimum temperature rise of the same group. From the temperature rise we can drawn that under the same working conditions, when the fluid inlet velocity is 50 m / s, the effect of heat transfer fluid-solid is the best. 

The use of alternative heat transfer fluid to water is often discounted based on assumptions, rather than proper technical analysis. As shown in the previous section, most of the commonly available heat transfer fluids can safely be used for most furnace applications, when combined with advanced cooler designs such as those pictured in Figures 2 and 5, and high fluid circulating velocities. Some coolants, like liquid metals, significantly improve on the performance of water as a heat transfer media, even at greatly reduced circulating velocities. 

Laminar Flow Although heat-transfer coefficients for laminar flow are considerably smaller than for turbulent flow, it is sometimes necessary to accept lower heat transfer in order to reduce pumping costs. The heat-flow mechanism in purely laminar flow is conduction. The rate of heat flow between the walls of the conduit and the fluid flowing in it can be obtained analytically. But to obtain a solution it is necessary to know or assume the velocity distribution in the conduit. In fully developed laminar flow without heat transfer, the velocity distribution at any cross section has the shape of a parabola. The velocity profile in laminar flow usually becomes fully established much more rapidly than the temperature profile. Heat-transfer equations based on the assumption of a parabolic velocity distribution will therefore not introduce serious errors for viscous fluids flowing in long ducts, if they are modified to account for effects caused by the variation of the viscosity due to the temperature gradient. The equation below can be used to predict heat transfer in laminar flow. 

Figure 18 Heat transfer coefficients as a function of gas velocity at different pressures in a three-phase fluidized bed (nitrogen-Paratherm NF heat transfer fluid-2.1 mm glass beads. (From Luo et al., 1997a.)...

Figure 19 Effect of pressure and gas velocity on heat transfer coefficients in a slurry bubble column (nitrogen-Paratherm NF heat transfer fluid-53 pm glass beads). (From Yang et al., 2000b.)...

By assuming a reasonable fluid velocity, together with fluid physical properties, standard heat transfer correlations can be used. 

Convection Heat Transfer. Convective heat transfer occurs when heat is transferred from a soHd surface to a moving fluid owing to the temperature difference between the soHd and fluid. Convective heat transfer depends on several factors, such as temperature difference between soHd and fluid, fluid velocity, fluid thermal conductivity, turbulence level of the moving fluid, surface roughness of the soHd surface, etc. Owing to the complex nature of convective heat transfer, experimental tests are often needed to determine the convective heat-transfer performance of a given system. Such experimental data are often presented in the form of dimensionless correlations. 

In the forced convection heat transfer, the heat-transfer coefficient, mainly depends on the fluid velocity because the contribution from natural convection is negligibly small. The dependence of the heat-transfer coefficient, on fluid velocity, which has been observed empirically (1—3), for laminar flow inside tubes, is h for turbulent flow inside tubes, h and for flow outside tubes, h. Flow may be classified as laminar or... 

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