Radiation, heat flow

In wall heat balance Eqs. (8.14) and (8.15), the radiation heat flows T and V from the heat sources and V 0 y2i from upper zone wall surfaces to lower zone wall surfaces are assumed to increase the temperature of the walls. In practical cases it is quite complicated to determine how much of the radiation flow rate will be distributed to outer walls and to other surfaces. 

Heat Transfer - The flow of heat from one area to another by conduction, convection, and/or radiation. Heat flows naturally from a warmer to a cooler material or space. 

Radiation heat flow between two surfaces or between a surface and its surroundings is governed by the Stefan-Boltzman equation providing the (nonlinear) radiation heat transfer in the form of... 

For purposes of physical reasoning, convection and radiation heat flow can be viewed as being represented by (nonlinear) thermal resistances, convective 9 and radiational 9r, respectively,... 

The radiation heat flow per 1 m s of the service area computed by equations ... 

Heat flow to the environment through conduction Heat flow to the environment through radiation Heat flow to the environment through convection Totai heat flow that dissipates into the environment... 

We can now begin to see how the 300 K radiation heat flow into LNG can be reduced by a large factor of 500-1000 by using a set of low emissivity baffles, or an equivalent multi-layer aluminium, suspended deck, each cooled by the cold vapour to a temperature of 150 K or less, above the liquid surface. 

The disc baffles work in the following way. The downward radiation heat flow from the top of the tank or container is partially absorbed and partially reflected back. The baffles are in turn cooled by increasing the enthalpy of the cold vapour. For each baffle, the radiative heating is balanced by the vapour cooling. In this way, with a series of baffles, the ambient radiation heat flow is almost completely stopped from entering the liquid and contributing to the liquid evaporation. 

Hollow plastic balls, 10-20 mm diameter, either plain or aluminised, are commonly used to reduce evaporation of volatile liquids in storage tanks at ambient temperatures. They may also be used to absorb radiation heat flows into cryogenic liquids. 

Vacuum Radiation Furnaces. Vacuum furnaces are used where the work can be satisfactorily processed only in a vacuum or in a protective atmosphere. Most vacuum furnaces use molybdenum heating elements. Because all heat transfer is by radiation, metal radiation shields ate used to reduce heat transfer to the furnace casing. The casing is water-cooled and a sufficient number of radiation shields between the inner cavity and the casing reduce the heat flow to the casing to a reasonable level. These shields are substitutes for the insulating refractories used in other furnaces. 

When a damp cloth is laid in an air flow, it settles after a certain time ic an equilibrium temperature, the so-called wet bulb temperature (0 ), which is determined through heat and mass transfer. Negotiating the heat flow obtained by radiation and conduction, the heat balance of the wet cloth in a stationary situation can be expressed as... 

For conduction the heat resistance is the distance divided by the heat conductivity, R = 8/X.A, and the heat conductance is heat conductivity divided by distance, U = X.A/8. For convection and radiation the heat resistance is 1 divided by the heat transfer factor, 1/aA, and the heat conductance is the same as the heat transfer factor, U aA. A coefficient of heat flow is also used, the K value, which is the total conductance ... 

Contact temperature measurement is based on a sensor or a probe, which is in direct contact with the fluid or material. A basic factor to understand is that in using the contact measurement principle, the result of measurement is the temperature of the measurement sensor itself. In unfavorable situations, the sensor temperature is not necessarily close to the fluid or material temperature, which is the point of interest. The reason for this is that the sensor usually has a heat transfer connection with other surrounding temperatures by radiation, conduction, or convection, or a combination of these. As a consequence, heat flow to or from the sensor will influence the sensor temperature. The sensor temperature will stabilize to a level different from the measured medium temperature. The expressions radiation error and conduction error relate to the mode of heat transfer involved. Careful planning of the measurements will assist in avoiding these errors. 

Effective radiant heat flow The heat exchange by radiation between the walls of the enclosure and the human body, E,if, in W m-T... 

Many everyday heat flows, such as those through windows and walls, involve all three heat transfer mechanisms—conduction, convection, and radiation. In these situations, engineers often approximate the calculation of these heat flows using the concept of R values, or resistance to heat flow. The R value combines the effects of all three mechanisms into a single coefficient. 

Thermal insulation in use today generally affects the flow of heat by conduction, convection, or radiation. The extent to which a given type of insulation affects each mechanism varies. In many cases an insulation provides resistance to heat flow because it contains air, a relatively low thermal conductivity gas. Ill general, solids conduct heat the best, liquids are less conductive, and gases are relatively poor heat conductors. Heat can move across an evacuated space by radiation but not by convection or conduction. 

Engineering thermal design of heat transfer equipment is concerned with heat flow mechanisms of the following three types—simply or in combination (1) conduction, (2) convection, and (3) radiation. Shell and tube exchangers are concerned primarily with convection and conduction whereas heaters and furnaces involve convection and radiation. 

In order to perform effectively as an insulant a material must restrict heat flow by any (and preferably) all three methods of heat transfer. Most insulating materials adequately reduce conduction and convection elements by the cellular structure of the material. The radiation component is decreased by absorption into the body of the insulant and is further reduced by the application of bright foil outer facing to the product. 

Radiation transfer is largely eliminated when an insulant is placed in close contact with a hot surface. Radiation may penetrate an open-cell material but is rapidly absorbed within the immediate matrix and the energy changed to conductive or convective heat flow. It is also inhibited by the use of bright aluminum foil, either in the form of multi-corrugated sheets or as outer facing on conventional insulants. 

Solar radiation may fall on outside walls or roofs, raising the skin temperature, and this must he taken into account. Most cold stores are huilt within an outer envelope which protects them from the elements and from direct sunshine. In cases where the insulation itself is subject to solar radiation, an allowance of 5 K higher outside temperature should he taken. Heat load must he estimated through all surfaces including piping, ducts, fan casings, tank walls, etc., where heat flows inwards towards the cooled system. 

The rate of heat conduction is further complicated by the effect of sunshine onto the outside. Solar radiation reaches the earth s surface at a maximum intensity of about 0.9 kW/ m. The amount of this absorbed by a plane surface will depend on the absorption coefficient and the angle at which the radiation strikes. The angle of the sun s rays to a surface (see Figure 26.1) is always changing, so this must be estimated on an hour-to-hour basis. Various methods of reaching an estimate of heat flow are used, and the sol-air temperature (see CIBSE Guide, A5) provides a simplification of the factors involved. This, also, is subject to time lag as the heat passes through the surface. 

Transient computations of methane, ethane, and propane gas-jet diffusion flames in Ig and Oy have been performed using the numerical code developed by Katta [30,46], with a detailed reaction mechanism [47,48] (33 species and 112 elementary steps) for these fuels and a simple radiation heat-loss model [49], for the high fuel-flow condition. The results for methane and ethane can be obtained from earlier studies [44,45]. For propane. Figure 8.1.5 shows the calculated flame structure in Ig and Og. The variables on the right half include, velocity vectors (v), isotherms (T), total heat-release rate ( j), and the local equivalence ratio (( locai) while on the left half the total molar flux vectors of atomic hydrogen (M ), oxygen mole fraction oxygen consumption rate... 

The next most importtmt parameters in Czochralski growth of crystals are the heat flow and heat losses in the system. Actually, aU of the parameters (with the possible exception of 2 and 9) are strongly ciffected by the heat flow within the crystal-pulling system. A tj pical heat-flow pattern in a Czochralski sjretem involves both the crucible and the melt. The pattern of heat-flow is important but we will not expemd upon this topic here. Let it suffice to point out that heat-flow is set up in the melt by the direction of rotation of the cr5rstal being pulled. It is also ctffected by the upper surface of the melt and how well it is thermally insulated from its surroundings. The circular heat flow pattern causes the surface to radiate heat. The crystal also absorbs heat and re-radiates it... 

The variable gap method is a steady-state method, with the merit that transport of heat by radiation can be separated from the total heat flow ... 

---