Heat generation by absorption

Energy for product heating generated by absorption of electromagnetic rays... 

The source term Si represents the heat generated by absorption //a and by reaction //r, which can be calculated by... 

The above equations for heat transfer apply when there is no heat generation or absorption during the reaction, and the temperature difference between the solid and the gas phase can be simply defined tliroughout the reaction by a single value. Normally this is not the case, and due to the heat of the reaction(s) which occur tlrere will be a change in the average temperature with time. Furthermore, in tire case where a chemical reaction, such as the reduction of an oxide, occurs during the ascent of tire gas in the reactor, the heat transfer coefficient of the gas will vary with tire composition of tire gas phase. 

The first law of thermodynamics tells us that any process in which the internal energy of the system changes is accompanied by absorption or by evolution of heat. The class of chemical sensors discussed in this chapter uses the heat generated by a steady-state chemical reaction as the source of analytical information. 

Ultrasonic energy, generated at great expense in the transducer, should not be lost to absorption while in transit to the cavitation zone. For this reason, conformal transducers are usually the best choice for process transducers, in that they allow for the shortest distance from the transducer to the cavitation zone. On the other hand, for laboratory work, mirrors offer easy separation of the waste heat generated in the transducer from the heat generated by ultrasound in the solution under test. Thus during process development, when accurate determinations of acoustical output is desired, judicious use of mirrors and windows is often called for. 

The basic element in the flow system is the perfectly-mixed reactor. In the multi-reactor system heat and mass transfer operation (absorption, desorption, dissolution of solids, heat generation or absorption as well as heat interaction between the reactor and the surroundings etc.) as well as chemical reactions may occur simultaneously, or not. The processes are governed by Eqs.(5-8), (5-12), (5-16), (5-19), (5-23) and (5-25) in the following, on the basis of which transition probabilities are derived as well as the single step transition matrix. 

The fibre was fed into a 0.5 m length of water-filled PVDF tube, and light from a Nd.-Yag laser operating at 1.06 ym was launched into the fibre. The water transmitted the heat generated by the absorption loss processes to the tube wall, so producing a proportional electrical charge. Temperature rises as low as 2 y°C/s were detectable for an input power of 1W, which corresponded to a loss of less than 0.1 dB/km. 

When radiation falls on a solid body, a definite fraction p may be reflected and the remaining fraction 1 — p enters the solid to be either transmitted or absorbed. Most solids (other than glasses, certain plastics, quartz, and some minerals) absorb radiation of all wavelengths so readily that, except in thin sheets, the transmissivity T is zero, and all nonreflected radiation is completely absorbed in a thin surface layer of the solid. The absorption of radiation by an opaque solid is therefore a surface phenomenon, not a volume phenomenon, and the interior of the solid is not of interest in the absorption of radiation. The heat generated by the absorption can flow into or through the mass of an opaque solid only by conduction. 

In direct solar dryers the material to be dried is placed in an enclosure with a transparent cover or side panels. Heat is generated by absorption of solar radiation on the product itself as well as on the internal surfaces of the drying chamber. This heat evaporates the moisture from the drying product. In addition it serves to heat and expand the air, causing the removal of the moisture by the circulation of air. 

---