Defined heat loss

The continuous current rating of a bus system can be defined by the current at which a steady-state thermal condition can be reached. It is a balance between the enclosure and the conductor s heat gain and heat loss. If this temperature is more than the permissible steady-state thermal limit it must be reduced to the desired level by increasing the size of the conductor or the enclosure or both, or by adopting forced cooling. Otherwise the rating of the bus system will have to be reduced accordingly. 

Clothing affects heat and moisture loss. Increasing the thickness or number of layers of clothing increases its insulating capability and reduces body heat loss. Clothing insulation is usually described with the do unit. Originally, t do was defined as the thermal resistance necessary for comfort while sedentary in a uniform still air environment of 21 °C. In conventional SI nomenclature I do has a thermal resistance of 0.155 K m-/W. Some ensembles do values and associated comfort temperatures are shown in Fig. 5.4. 

The effect of the heat losses to the inlet on the thermal states of the micro-channel depends mainly on the meniscus position, which is determined by the flow parameters. To characterize this effect, the coefficient of efficiency is introduced it may be defined as the ratio of the energy expended to the liquid vaporization and the total energy supplied to the micro-channel. 

The position of the meniscus within the micro-channel defines the type of temperature distribution. In the first case, when the meniscus is near the outlet, the temperature gradient of the vapor region is small. The rate of evaporation is determined mainly by the heat flux in the liquid region. Therefore, the necessary condition of the evaporation consists of the existence of the region (near the meniscus), where the water is overheated (its temperature is higher than the temperature of boiling). The heat losses to the inlet tank cause the existence of the temperature maximum. 

It is also well known that there exist different extinction modes in the presence of radiative heat loss (RHL) from the stretched premixed flame (e.g.. Refs. [8-13]). When RHL is included, the radiative flames can behave differently from the adiabatic ones, both qualitatively and quantitatively. Figure 6.3.1 shows the computed maximum flame temperature as a function of the stretch rate xfor lean counterflow methane/air flames of equivalence ratio (j) = 0.455, with and without RHL. The stretch rate in this case is defined as the negative maximum of the local axial-velocity gradient ahead of the thermal mixing layer. For the lean methane/air flames,... 

Generally, the efficiency of steam turbines decreases with decreasing load. The overall turbine efficiency can be represented by two components the isentropic efficiency and the mechanical efficiency. The mechanical efficiency reflects the efficiency with which the energy that is extracted from steam is transformed into useful power and accounts for mechanical frictional losses, heat losses, and so on. The mechanical efficiency is high (typically 0.95 to 0.99)6. However, the mechanical efficiency does not reflect the efficiency with which energy is extracted from steam. This is characterized by the isentropic efficiency introduced in Figure 2.1 and Equation 2.3, defined as ... 

The minus sign stems from the fact that heat loss is defined as negative and heat gain as positive. Without the minus sign, we would have a positive quantity equal to a negative quantity. 

The characteristic heat loss time is generally defined as the time it takes to cool the gas from the temperature (T - T0) to f/ - T0)le] and is found to be... 

When Pe is less than about 46, heat loss effects on Umay be appreciable. If an analogy is made with turbulent flames using a turbulence Peclet number defined by Pe = Urd/aT, where ar 0.058RciZ/ [21, 22], it is readily shown that Pe is substantially above 46 for all of the present turbulent flame experiments. The influence of heat loss is therefore not likely a contributor to the bending effect exhibited in Fig. 15.4. 

When the compression process in the diffuser involves heat loss, the total enthalpy decreases from hoa to ho2- The enthalpy recovery factor, r, j, is defined as... 

For a reactor of given dimensions and a fixed value of T, that is a particular Qq line, there will be the family of Qq lines shown in Fig. 21. One of the Qq lines will be tangential to the heat loss line alternatively, one can think of a particular concentration which defines the Qg lines and varying either or the slope of the Qq line until the tangency condition is just fulfilled. This situation is shown in Fig. 21 and it corresponds to the condition where a steady reaction is Just possible any increase in the reactant concentration or the temperature of the surroundings (T ) or reduction in the rate of heat loss, will result in explosion. At the tangency point, the reactant temperature has a critical value Tc and also Qq = Qq, i.e. 

Prior to the tests, all the samples were dried in a vacuum oven at 80°C for at least 72 h to minimize the moisture effect and then transferred to a desiccator. Measurements were carried out on a cone calorimeter provided by the Dark Star Research Ltd., United Kingdom. To minimize the conduction heat losses to insulation and to provide well-defined boundary conditions for numerical analysis of these tests, a sample holder was constructed as reported in [14] with four layers (each layer is 3 mm thick) of Cotronic ceramic paper at the back of the sample and four layers at the sides. A schematic view of the sample holder is shown in Figure 19.12. Three external heat fluxes (40, 50, and 60kW/m2) were used with duplicated tests at each heat flux. 

Some specific studies on the measurement of heat losses and indoor temperatures in buildings deserve attention. In his review of the relative importance of thermal comfort in buildings, McIntyre considered that the mean radiant temperature was the most important parameter, followed closely by the "radiation vector," which is defined as the net radiant flux density vector at a given point and measures the asymmetry of the thermal radiation field in a room (97). Benzinger et al. characterized the mean radiant temperature, and asymmetric radiation fields, using a scanning plane radiometer, which maps the plane radiant temperature in a given space indoors (98). 

A measure of how efficiently a system can yield energy with minimum heat loss, we define the thermodynamic efficiency as the ratio of useful work output (over a complete cycle) divided by heat input ... 

The feed is a mixture of CH CO, CO H2, and N2 with poisoning H2S having been removed, and only a small amount of CO, present. The gas will be fed to a single bulk methanator at a pressure of 360 psia and temperature of 450°F. The feed rate for the initial gas brought into the system (before recycle gas is added) is 390 xlO6 standard cubic feet per day, where a standard cubic foot is defined as at 60°F and 1 atm (i.e., 1 lb mole = 379.5 standard cubic feet). The critical methanation reaction is CO + 3H2 = CH, + H20 with an exothermic heat of reaction of AH = — 95,404 Btu/lb mole CO. Ignore any heat effects due to CO, methanation reaction or overall heat losses. 

Afl of the adiabatic problem [defined by equation (5-45) for the adiabatic problem, for example] with a nondimensional function o (t), and the non-dimensional heat-loss rate is... 

The dependence L(T/) l/D implies that the left-hand side of equation (24) increases as the tube diameter D decreases. Since none of the other physical parameters in equation (24) depend on Z), an extinction condition is reached when the tube diameter D becomes sufficiently small. This minimum tube diameter is called the quenching diameter, which is empirically approximately the same as (perhaps 25 % or 50 % greater than) the quenching distance d defined in Section 8.1. Quenching phenomena can thus be attributed to conductive heat losses. 

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