Heat exchange coefficient

From Tolmin s theory and experimental data (e.g., Reichardtthe relationship between velocity profile and temperature profile in the jet cross-section can be expressed using an overall turbulent Prandtl number Pr = v /a, where Vf is a turbulent momentum exchange coefficient and a, is a turbulent heat exchange coefficient ... 

Convective heat exchange coefficient A complicated factor involving the surface ge... 

As the diameter is decreased, the heat transfer from a unit volume intensifies because of the increase in the ratio of surface to volume (d-1) heat exchange per unit surface also intensifies. For a constant value of the Nusselt number the heat exchange coefficient is proportional to d l. Under the rough assumption that Tc and E change little from one case to the next, we come to the conclusion that at the limit the Peclet number (numerically equal for gases to the Reynolds number), based on the flame velocity (or adiabatic flame velocity u0) has a specific value... 

In the case of significant change of the physical chemical properties of the reaction mixture, the overall heat exchange coefficient (U) will also be a function of time. The heat transfer properties are usually a function of temperature, where changes in the viscosity of the reaction mass play a dominant role. 

An often-used method for the limitation of the heat release rate is an interlock of the feed with the temperature of the reaction mass. This method consists of halting the feed when the temperature reaches a predefined limit. This feed control strategy keeps the reactor temperature under control even in the case of poor dynamic behavior of the reactor temperature control system, should the heat exchange coefficient be lowered (e.g. fouling crusts) or feed rate too high. 

The cooling capacities are calculated for a heat exchange coefficient of 500 Wm 2 K 1 and a temperature difference of 10 K with the cooling medium. 

Assume a heat exchange area of the reactor of 6 m2 and an overall heat exchange coefficient of 500 Wm 2 K"1. 

The first strategy appears to be very effective in fact, as shown by (2.30) and (2.31), the direct (linear) effect of increasing the flow rate is augmented by the increase of the jacket-side heat exchange coefficient. This control action can be realized without a noticeable time delay by a simple control valve, the only drawback for its quantitative assessment being the nonlinear relationship between the overall heat exchange coefficient and the flow rate of the heat exchange fluid. 

For many years I have carried around with me two sheets of notepaper that contain heat-exchange coefficient equations. These equations calculate the inside and outside tube wall heat-transfer coefficients for... 

Other prevalent issues concern the fact that most of the heat recovery for air preheating is performed on clean flue gases, which allows for higher heat exchange coefficients and lower heat exchange surfaces. 

The heat exchange coefficient between solid and gas is written as [3] ... 

Thermal properties of materials and heat exchange coefficients... 

The shap>e of the temperature profile in a flat slab having the following characteristics, thickness 2b and thermal conductivity k, and conditions, initial temperature Tj, suddenly exposed to a cooling medium at temperature To and draining heat from the slab with a heat exchange coefficient h it is determined by the dimentionless Biot number ... 

Where S is the slab surface, h heat transfer coefficient. To the coolant temperature and T the lumped sample temperature. By assuming that the heat exchange coefficient h is constant, then slope of the cooling rate versus temperature curve is also constant, while the slab temperature decays expenentially with time. 

Fig. 5. Heat exchange coefficient vs. coolant mass flux for four different spray nozzles...

The solution of equation (3), introducing the dimensionless temperature of the Cu-Be slab withboimdaiy conditions T=Tj for t=0 and constant heat exchange coefficient h, is ... 

The heat exchange coefficient of the entire structure is 1.1 W/m K, the total energy permeability factor is 17%, the transmission factor is 6%, the degree of reflection 34% and the light transmission factor 22% (source ... 

The existing heat exchange coefficient is smaller than the relevant coefficient for condensation water. The effect of condensation water is therefore irrelevant since the durability of the fagade is not affected. 

Adding up Equations (10) and (11) leads to a single heat exchange coefficient that combines radiation and convection and yields the total amount of heat exchanged with environment ... 

A least-squares optimization routine is employed to systematically adapt the heat capacity, thermal conductivity, and convective heat exchange coefficient of the simulation model until a good agreement between measured and simulated impedance spectra is achieved. Final values of the parameter variation process represent the thermal parameters of the real battery. Figure 8 compares an impedance spectrum from measurement data with an impedance spectrum derived from the result values of the optimization process. As a good agreement... 

From this equilibrium, the heat exchange coefficient can be calculated after transformation of Equation (12) ... 

The heat exchange coefficient is then removed from the parameter identification process, so that only heat capacity and thermal conductivity remain for identification. 

By providing a predefined value for the heat exchange coefficient, fewer low frequencies are required for precise parameter identification and TIS measurements can be shortened. However, the separate measurement of the heat exchange coefficient requires additional measurement time of about three hours for an 18650 Li-ion cell. Consequently, this acceleration approach is particularly advantageous, when several cells of the same type are investigated, as the heat exchange coefficient has to be determined only once. 

Using direct signal components to separately determine the heat exchange with environment saves time for additional measurements. It also provides a precise value for the heat exchange coefficient and, thus, reduces the number of low frequencies required for the parameter identification process. A minimum frequency of 1 mHz instead of 0.1 mHz reduces measurement time to less than 5 hours. 

In addition to an optimization of the set of examined frequencies and the number of oscillations, beneficial effects of a separate determination of the heat exchange with environment were identified. Either by an additional heating measurement or by an evaluation of direct signal components, a precise identification of the heat exchange coefficient is possible. This reduces markedly the required range of frequencies for the accurate identification of heat capacity and thermal conductivity. 

The heat released in every zone, depending on the polymerisation rate and amount of a monomer reacting in the reaction zone is small, and heat exchange coefficients are such as to meet the requirement of temperature uniformity in every reaction zone. In accordance with this condition in the reaction zone, the polymer should be formed with a MWD and MW corresponding to the temperature determined by the heat balance. Calculations show that such conditions are performed for the following equation [3] ... 

This situation demands an extremely attentive determination of the input data in the calculation (heat exchange coefficients, area of the surfaces exposed to the atmosphere and so on) to ensure that the various heat quantities exchanged by the mixture are evaluated in a conservative way. The following looks at some input data for the calculation whose determination is usually uncertain. 

Active heat exchange or mixing elements like fins or static mixers [24] this would increase the overall heat exchange coefficient. However, the gained benefits should not increase the technical complexity. 

Better stability and higher heat capacity partial vaporization is possible better heat exchange coefficient... 

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