Heat flow, by radiation

Radiation may be important in heat transfer to outside tube surfaces. Inside tubes, the surface cannot see surfaces other than the inside wall of the same tube, and heat flow by radiation does not occur. Outside tube surfaces, however, are necessarily in sight of external surfaces, if not nearby, at least at a distance, and the surrounding surfaces may be appreciably hotter or cooler than the tube wall. Heat flow by radiation, especially when the fluid is a gas, is appreciable in comparison with heat flow by conduction and convection. The total heat flow is then a sura of two independent flows, one by radiation and the other by conduction and convection. The relations given in the remainder of this section have to do with conduction and convection only. Radiation, as such and in combination with conduction and convection, is discussed in Chap. 14. 

To measure the wet-bulb temperature with precision, three precautions are necessary (1) the wick must be completely wet so no dry areas of the wick are in contact with the gas (2) the velocity of the gas should be large enough to ensure that the rate of heat flow by radiation from warmer surroundings to the bulb is negligible in comparison with the rate of sensible heat flow by conduction and convection from the gas to the bulb (3) if makeup liquid is supplied to the bulb, it should be at the wet-bulb temperature. When these precautions are taken, the wet-bulb temperature is independent of gas velocity over a wide range of flow rates. 

Let us now outline the various heat flows by radiation, conduction and convection respectively, distinguishing between the A heat in-flows and the B heat in-flows. The next chapter. Chap. 3, will go on to describe, in detail, the techniques available for controlling and reducing the heat flows. 

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... 

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. 

A bare thermocouple is used to measure the temperature of a gas flowing through a hot pipe. The heat transfer coefficient bet ween the gas and the thermocouple is proportional to the 0.8 power of the gas velocity and the heat transfer by radiation from the walls to the thermocouple is proportional to the temperature difference. 

Heat Flux - The rate of heat transfer per unit area normal to the direction of heat flow. It is the total heat transmitted by radiation, conduction, and convection. 

Heat transfer by radiation only. The figure shows only the order of magnitude. Density of heat flow (q) as a function of 7"str2 (ice or product surface temperature) at three temperatures 7"strl of the radiation surface (+100, +50, +30 °C) as parameter... 

In gas-filled windows there are three heat transfer mechanisms conduction and convection through the gas layer and radiation between the surroundings and the glass surfaces. The heat flow by conduction is minimized by using a fairly thick gas layer with a low conductivity. With even thicker layers, the effect of convection becomes important. Conduction and radiation cause similar heat fluxes, with heat transfer coefficients of a few watts per square metre per kelvin. 

In many applications heat transfer by convection must be considered in addition to radiative heat transfer. This is, for example, the case where a radiator releases heat to a room which is at a lower temperature. Radiative heat exchange takes place between the radiator and the walls of the room, whilst at the same time heat is transferred to the air by convection. These two kinds of heat transfer are parallel to each other and so the heat flow by convection and that by radiation are added together in order to find the total heat exchanged. The heat flux then becomes... 

Thermal radiation differs from heat conduction and convective heat transfer in its fundamental laws. Heat transfer by radiation does not require the presence of matter electromagnetic waves also transfer energy in empty space. Temperature gradients or differences are not decisive for the transferred flow of heat, rather the difference in the fourth power of the thermodynamic (absolute) temperatures of the bodies between which heat is to be transferred by radiation is definitive. In addition, the energy radiated by a body is distributed differently over the single regions of the spectrum. This wavelength dependence of the radiation must be taken as much into account as the distribution over the different directions in space. 

The space between the two tanks is filled with layers of thin aluminized plastic film separated by a lightweight coarse plastic screen. These serve as a shield against the passage of thermal radiation from the outer to the inner tank. The air between the tanks and around the insulation is removed with a vacuum pump. The high vacuum serves to stop heat flow by conduction. The liquid fill and gas withdrawal lines are coaxial that is, one inside the other. They are made from materials with low thermal conductivity and are coiled inside the insulation to minimize heat flow down the length of the pipe from the outside into the inner tank. 

When bodies at different temperatures are placed in sight of one another inside an enclosure, the hotter bodies lose energy by emission of radiation faster than they receive energy by absorption of radiation from the cooler bodies, and the temperatures of the hotter bodies decrease. Simultaneously the cooler bodies absorb energy from the hotter ones faster than they emit energy, and the temperatures of the cooler bodies increase. The process reaches equilibrium when all the bodies reach the same temperature, just as in heat flow by conduction and convection. The conversion of radiation into heat on absorption and the attainment of temperature equilibrium through the net transfer of radiation justify the usual practice of calling radiation heat. ... 

The heater consists of a coil of Nichrome wire (150 Q) wound upon a mica former. Two coils of platinum wire are attached to the ends of the heater and leads from these go to the electrical outlets. The coils serve to prevent heat leaks away from the heater as the incoming vapour, at temperature Ti, passes over them before it reaches the heater. During experiments the calorimeter heater is supplied from the same stabilized potential somce as the boiler heater so that the temperature rise of the vapour at the heater is almost independent of flow rate. A baffle situated between T1 and H2 serves to prevent heat transfer by radiation from the heater to the thermometer. 

Heat Flux the rate of heat transfer per unit area normal to the direction of heat flow. It is the total heat transmitted by radiation, conduction, and convection HIPS an acronym for High Integrity Protective System. Is a set of components, such as sensors, logic solvers, and final control elements (e.g., valves), arranged for the purpose of reverting a process to a safe state when predetermined conditions are violated. Sometimes also referred to as HIPPS, High Integrity Pressure Protective System... 

We measured the thermal conductivity of the hydrophobic silica aerogel, prepared as described above. We used the thermal flow method, under conditions of constant flow, in accordance with ASTM-C518, and used the thermal conductivity measuring system shown in Figure 13-6. We carried out the measurements at 40°C and 20°C, on the high and low temperature plate respectively, so that heat transfer by radiation could be ignored. 

Since the amount of heat transferred by radiation varies with the fourth power of the absolute flame temperature, the greatest efficiency will always be realized with maximum flame temperature. But the distribution of the heat is also important. Inereasing the amount of exeess air will reduce the flame temperature, thereby redueing the heat transfer rate in the vicinity of the burner. Since the net flow of thermal power into the system has not changed, the rate of heat transfer farther away from the burner tends to increase. 

For a square channel, is the length of the channel side. In the absence of reactions and heat transfer by radiation, and when laminar flow is fully developed, the Sherwood and Nusselt... 

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 ... 

Some of the issues of clothing comfort that are most readily involve the mechanisms by which clothing materials influence heat and moisture transfer from skin to the environment. Heat flow by conduction, convection, and radiation and moisture transfer by vapor diffusion are the most important mechanisms in very cool or warm environments from the skin. 

The basic heat transfer mechanisms are conduction, convection, and radiation. The heat transfer can be achieved through direct or indirect contact of streams. It can happen between two streams or multiple streams, (n this chapter, we will concentrate on heat transfer through conduction and convection, indirect conlBct, and between two flow streams. Direct contact heat transfer equipment such as cooling tower and heat transfer by radiation... 

Dewar s flask works by cutting out heat transfer, from bofli A and B heat flows, by conduction and convection with the use of a vacuum in a low fliermal conductivity vessel, and reducing radiation by the use of low emissivity reflecting films of silver. It still represents a standard for today, by addressing all flie insulation techniques required to minimise boil-off. 

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. 

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

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