Heat transfer velocities

The Nusselt number, Nu = hl/k, that determine the ratio between the effective heat transfer velocity and the molecular conductivity (i.e., a dimensionless temperature gradient at the interface). 

Infrared observations of the effects of surface films on heat and gas transfer have been made in situ (Frew et al. 2004). Figure 10 (upper) shows the instrumented air-sea interaction catamaran LADAS operating within a banded surface slick during the CoOP97 study. Simultaneous infrared imaging and measurements of wave slope and surfactants outside and inside of the slick determined that, when the slick was entered, the surface temperature distribution shifted toward lower temperatures and the spatial scales of the temperature fluctuations at the sea surface increased (Fig. 10, lower), characteristic of reduced surface renewal and an attenuation of mixing in the aqueous boundary layer. The estimated temperature gradient increased from 0.13 Kelvin to 0.24 Kelvin and the net heat flux dropped from 77.2 Watt m 2 to 36.5 Watt m"2. The 80% increase in AT and a 50% decrease in the heat flux lead to a decrease in the estimated heat transfer velocity from 49.7 cm h"1 to 13.1 cm h"1. 

Schmidt number scaling of the heat transfer velocities according to Eq. 1.2 suggests nearly an order of magnitude reduction in the gas transfer velocity due to the slick (5.1 cm h 1 and 0.64 cm h"1 outside and inside the slick respectively) at a wind speed of 4 m s 1. 

Studies of the air-water gas exchange process by infrared imaging techniques enable local and fast measurements of the heat transfer velocity. Applying Schmidt number scaling, the obtained gas transfer velocities are consistent with mass balance methods and are in good agreement for the... 

Select a convection tube arrangement that will give a maximum flue gas mass velocity of about 0.3 to 0.4 pounds per square foot per second. Calculate the mass velocity, G. For precise ratings, calculate the gas film coefficient using Figures 1-12, 1-13 and 1-14 and equations for f, the factor for wall radiation in the convection section and he, the convective heat transfer coefficient. Calculate the in tube coefficient using any applicable method. Then calculate the overall transfer coefficient from the equation for Ue, the convective heat transfer velocity. 

Let us consider how large is the heat transfer capacity of the former adsorber-desorber construction. The heat conductivity of a thin copper layer is larger at least with two magnitudes than the carbon or other adsorbents layers ones. Thus the governing transfer properties are the conductivities of adsorbent layers. The thermal conductivity of a carbon powder -0.6 kJ/m hr K. The average thickness is 0.1-0.2 mm. Using these data the technically possible heat transfer velocity... 

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

In the Couette flow inside a cone-and-plate viscometer the circumferential velocity at any given radial position is approximately a linear function of the vertical coordinate. Therefore the shear rate corresponding to this component is almost constant. The heat generation term in Equation (5.25) is hence nearly constant. Furthermore, in uniform Couette regime the convection term is also zero and all of the heat transfer is due to conduction. For very large conductivity coefficients the heat conduction will be very fast and the temperature profile will... 

Fundamental models correctly predict that for Group A particles, the conductive heat transfer is much greater than the convective heat transfer. For Group B and D particles, the gas convective heat transfer predominates as the particle surface area decreases. Figure 11 demonstrates how heat transfer varies with pressure and velocity for the different types of particles (23). As superficial velocity increases, there is a sudden jump in the heat-transfer coefficient as gas velocity exceeds and the bed becomes fluidized. 

Equipment. Food freezing equipment can be classified by the method and medium of heat transfer used. High velocity air is the most... 

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... 

The effective thermal conductivity of a Hquid—soHd suspension has been reported to be (46) larger than that of a pure Hquid. The phenomenon was attributed to the microconvection around soHd particles, resulting in an increased convective heat-transfer coefficient. For example, a 30-fold increase in the effective thermal conductivity and a 10-fold increase in the heat-transfer coefficient were predicted for a 30% suspension of 1-mm particles in a 10-mm diameter pipe at an average velocity of 10 m/s (45). 

The minimum velocity requited to maintain fully developed turbulent flow, assumed to occur at Reynolds number (R ) of 8000, is inside a 16-mm inner diameter tube. The physical property contribution to the heat-transfer coefficient inside and outside the tubes are based on the following correlations (39) ... 

R. A. Smith, "Economic Velocity in Heat Exchangers," ASME/AIChE 20th National Heat Transfer Conference, Milwaukee, Wis., 1981. 

Static mixing of gas—Hquid systems can provide good interphase contacting for mass transfer and heat transfer. Specific interfacial area for the SMV (Koch/Sulzer) mixer is related to gas velocity and gas holdup ( ) by the following ... 

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