Heat convection-heating products

Calculate maximum air velocity, airflow rate, and excessive temperature (relative to the ambient air temperature equal to 20 °C) in thermal plume above the heated cube (0.66 m x 0.66 m x 0.66 m) with convective heat production = 225 W, at heights of 2.0 m and 4.0 m above the floor level. Neglect temperature gradient along the room height. Compare the results with predictions made for the same case using CFD code (Fig. 7.80). 

The energy that powers terrestrial processes is derived primarily from the sun and from the Earth s internal heat production (mostly radioactive decay). Solar energy drives atmospheric motions, ocean circulation (tidal energy is minor), the hydrologic cycle, and photosynthesis. The Earth s internal heat drives convection that is largely manifested at the Earth s surface by the characteristic deformation and volcanism associated with plate tectonics, and by the hotspot volcanism associated with rising plumes of especially hot mantle material. 

In Eq. (13), r stands for the production (or consumption) of the species of interest due to a chemical reaction, while in Eq. (12) q represents the heat production, e.g., due to one of more chemical reactions. Equation (13) is often referred to as the Convection-Diffusion-Reaction (CDR) equation. 

If these processes produce too much heat, the body attempts to lose heat by vasodilation within the skin (via convection) and sweating (via evaporation of the water in the sweat). Both are well-known characteristics of fever. The patient s experience of alternate shivering and sweating (so well described by Hippocrates) probably represents an impairment of the thermorequlatory centre in the hypothalamus that regulates the balance between heat loss and heat production, resulting in fluctuations in body temperature. 

Fig. 15. The possible mechanisms by which a strong electric field can affect cells in suspension or adherently growing. Most of the heat is produced near the electrodes and, therefore, tends not to be a direct problem as it can be easily dissipated into the substrate. This heating can, however, induce convection currents which, in turn, may impose mechanical stress on an adherent cell. There is also some heating between the electrodes. At low frequencies, this occurs only in the medium although it may be concentrated in regions surrounding the cell. At high frequencies, this heating becomes more uniform but, because high frequency currents can flow inside the cell, there is some internal heat production. The total amount of heat evolved depends on the conductivity of the medium and on the square of the applied voltage.

At low cirabient temperatures a greater portion of the metabolic heat production (depending upon exercise intensity and clothing) is dissipated by convection and radiation and a minor portion by evaporation of sweat and respiratory water. As ambient temperature rises, the portion of heat dissipated by convection and radiation decreases progressively in concert with a proportional increase in the rate of sweating and evaporative heat loss. The coordination of the rate of heat loss between conduction, radiation, and evaporation is so precise that, for ambient dry-bulb temperatures between 5 C and 29 C, the equilibrium level of core (rectal) temperature is related directly to the intensity of the exercise load and is independent of environmental temperature (25). 

The temperature distribution in a reacting mixture is stabilized when the rate of loss of heat by conduction or convection from any volume element is equal to that produced by the reaction itself in that volume element. In the case that the rate of heat loss cannot compensate for the rate of heat production, a stationary or quasi-stationary temperature distribution is impossible and the temperature of the reaction mixture increases exponentially, causing the reaction rate to do likewise, and a thermal explosion results. This is illustrated in Fig. XIV. 1, which follows... 

As a possible radiogenic heat source in the Earth, the presence of an appreciable amount of potassium in the core was suggested over 30 years ago (Lewis, 1971 Hall and Murthy, 1971). The estimated concentration of potassium in the mantle is 240 ppm (McDonough and Sun, 1995). If the concentration of potassium in the core is at a comparable level, the present-day heat production due to would be on the order of 10 W, enough to drive the geodynamo. Radiogenic heat production due to also has direct implications for convection in the outer core, heat flux at the CMB, and the dynamics of the lower mantle. Recent studies of plume dynamics and considerations of the age of the inner core have inspired a renewed interest in the potassium content of the core (Murthy et ai, 2003, and references therein see Chapter 2.15). 

A problem that often arises in determining mobilities is heat production. The current passing through a cell develops heat, and this not only causes convection currents, which change the mobilities, but also increases the rate of evaporation of the buffer solution when strip electrophoresis is employed. This loss of buffer causes a change in mobility because of changes in the potential and current it also causes a capillary action in the paper or gel, because it will take up solution from the buffer tank. The usual maximum amount of heat that can be tolerated is 0.15 watts/cm . The heat produced can be calculated from the equation ... 

Horizontal Fluid Layers. A uniform volumetric heat production q " in a horizontal layer bounded above by an isothermal surface and on the sides and bottom by adiabatic surfaces is depicted in Fig. 4.40. For a stationary fluid, the Nusselt number defined in the figure is Nu = 2, and the temperature difference used to construct the Rayleigh number is T0 - 7] = q" L l2k. As Ra increases from zero, the layer remains stable and heat flow is by conduction until a critical Rayleigh number of 1386 is reached [167]. Thereafter convection promotes a monotonic increase in Nu with Ra. For water (2.5 < Pr < 7), and for Ra < 10 2, the heat transfer data of Kulacki et al. [166-168] are accurately represented by... 

If the mean surface temperature is independent of metabolic rate, a heat exchange mechanism other than convection or radiation must be responsible for removing excess heat generated when a subject exercises. One s attention turns immediately to sweating, a phenomenon which has been studied in many laboratories. For subjects in equilibrium with the environment, the sweat rate is directly proportional to the total heat production rate. Perhaps this is expected because regulating central temperature by controlling the blood flow rate to skin, and, thus, the skin temperature is relatively ineffective. As the skin temperature rises, the difference between the central temperature and the skin temperature... 

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