Net radiation flow

The net radiation flow transferred by direct radiative interchange from 1 to 2 is therefore... 

The heat flow supplied to zone i from outside (or, with Q < 0, released to the outside), is the sum of the net radiation flows y from (5.134) between zone i and the other zones that bound the enclosure. 

As comparison with (5.134) shows, the heat flow Qi transferred from 1 to 2 is increased compared to the net radiation flow )2 due to the reradiating walls, because Fi2 > Fi2. If the radiation source and receiver have flat or convex surfaces (Fn = 0, F22 = 0), then the view factors Fir and F2r can lead back to F 2 and instead of (5.140)... 

The rectifier, or diode, is an electronic device that allows current to flow in only one direction. There is low resistance to current flow in one direction, called the forward bias, and a high resistance to current flow in the opposite direction, known as the reverse bias. The operation of a pn rectifying junction is shown in Figure 6.17. If initially there is no electric field across the junction, no net current flows across the junction under thermal equilibrium conditions (Figure 6.17a). Holes are the dominant carriers on the / -side, and electrons predominate on the n-side. This is a dynamic equilibrium Holes and conduction electrons are being formed due to thermal agitation. When a hole and an electron meet at the interface, they recombine with the simultaneous emission of radiation photons. This causes a small flow of holes from the jp-region... 

The second material property is heat of gasification, L, defined as the net heat flow into the material required to convert one unit mass of solid material to volatiles. The net heat flux into the material can be obtained from an energy balance at the surface of the specimen. Typically, a sample exposed in a bench-scale calorimeter is heated by external heaters and by its own flame. Heat is lost from the surface in the form of radiation. Owing to the small sample size, the flame flux is primarily convective, and flame absorption of external heater and specimen surface radiation can be neglected. Hence, L can be defined as... 

The energy balance equations for all the zones need to be established to solve this radiative exchange problem. This is done using the net-radiation method introduced by G. Poljak [5.49], This yields a system of linear equations that, when solved, deliver the unknown temperatures and heat flows. With simple... 

An enclosure surrounded by three isothermal surfaces (zones), like that shown schematically in Fig. 5.59, serves as a good approximation for complicated cases of radiative exchange. Zone 1 at temperature 7 and with emissivity is the (net-) radiation source, it is supplied with a heat flow Q1 from outside. Zone 2 with temperature T2 < Tx and emissivity e2 is the radiation receiver, whilst the third zone at temperature TR, assumed to be spatially constant, is a reradiating wall, (Qr = 0). The heat flow Qi = — Q2 transferred by radiative exchange in the enclosure is to be determined. 

One of the oldest and best-known methods of determining the humidity of a gas is to measure its wet bulb temperature and its dry bulb temperature. The wet bulb temperature is the steady temperature reached by a small amount of liquid evaporating into a large amount of rapidly moving unsaturated vapor-gas mixture. It is measured by passing the gas rapidly past a thermometer bulb kept wet by a saturated wick and shielded from the effects of radiation. If the gas is unsaturated, some liquid is evaporated from the wick into the gas stream, carrying with it the associated latent heat. This latent heat is taken from within the liquid in the wick, and the wick is cooled. As the temperature of the wick is lowered, sensible heat is transferred by convection from the gas stream and by radiation from the surroundings. At steady state, the net heat flow to the wick is zero and the temperature is constant. 

In Eq. (10.50) Cp is the heat capacity of the liquid in J/(kg K), m the mass of liquid contained in the pool in kg, Fq the surface area of the pool in m, qy the enthalpy loss by vaporization, qg the heat transfer from the ground, q the convective heat transfer from the air and q the net radiation gain (pool—environment), all of them in W/m riiz Anally is the mass flow rate into the pool in kg/s and hf,z — hf,p the difference of the enthalpy of the spilled liquid and that already present in the pool in J/kg. 

In the cavities of masonry walls, the transmission of heat is by radiation and convection. The thermal inertia of the air is assumed to be neghgible, and the net heat flow into the cavity is given by the following expression ... 

In the gamma heating phase, there are a number of technical concerns that need to be addressed as indicated below and the reference analysis for power limitations must be documented. Due to the complexities and uncertainties associated with this problem (e. g., convection with limited net air flow radiation between near white bodies with low and time variant emissivities ... 

The radiation network of this two-surface enclosure consists of two surface resistances and one space resistance, as shown in Fig. 13-24. hi an electrical network, the electric current flowing through these resistances connected in series would be determined by dividing the potential difference between points A and B by the total resistance between the same two points. The net rate of radiation transfer is determined in the same manner and is expressed as... 

The net flow of heat Q, from the radiator to the surroundings enclosing it is... 

The physical meaning of the terms in the enthalpy equation can be identified from the above modeling analysis. The term on the LHS denotes the rate of accumulation of enthalpy within the control volume per unit volume the first term on the RHS denotes the net rate of increase of enthalpy by convection per unit volume the second term on the RHS, that is already known from the foregoing discussion, denotes the rate of increase of enthalpy by the heat flow (e.g., conduction, inter-diffusion effects, Dufour effects and radiation) per unit volume the third term on the RHS denotes the rate of work done by the pressure, which is induced by the surrounding fluid motion, acting on the mixture within the control volume per unit volume the last two terms are already known from the foregoing discussion, nevertheless the fourth term on the RHS denotes the irreversible rate of increase of enthalpy by viscous dissipation per unit volume the fifth term on the RHS denotes the rate of work done by external body forces acting on the mixture within control volume. 

NATURE OF HEAT FLOW. When two objects at different temperatures are brought into thermal contact, heat flows from the object at the higher temperature to that at the lower temperature. The net flow is always in the direction of the temperature decrease. The mechanisms by which the heat may flow are three conduction, convection, and radiation. 

---