Heat, cycle convection

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. 

Solar activity cycle—The periodic, roughly 11-year rise and fall in the number of active features, such as sunspots, prominences, and flares, in the Sun s atmosphere it is thought to be caused by the periodic tangling of the Sun s magnetic field by its rotation and the motion of heat-transporting convective bubbles of gas beneath its surface. 

The first thermobalance was probably the instrument described by Honda (30) in 1915. This instrument, as shown in Figure 3.18, consisted of a balance with a quartz beam. The sample was placed in a porcelain or magnesia crucible, G, which was suspended in an electrically heated furnace, J. Attached to the opposite end of the balance beam was a thin steel wire helix, E, which was immersed in oil contained in a Dewar flask, H. The Dewar-flask-helix assembly was adjusted by a screw mechanism to maintain the balance beam in a null position. A rather low heating rate was employed, since it took 10-14 hours to attain a temperature of 1000°C. However. Honda used a quasi-isothermal heating cycle in that during a mass-loss transition the furnace temperature was maintained at a constant temperature until the transition was completed. This procedure alone sometimes required 1-4 hours. Convection currents were evident above 300°C, as might be expected from the furnace sample arrangement. A sample mass of about 0.6 g was normally employed. 

In stead of a liquid on-off mode, a high-low liquid feed cycle may be applied. This may be beneficial for the elimination of (some of the) heat by convection (and possibly evaporation) and deliverance of liquid-phase reactant to the location of reaction. Also, for the above mentioned example, styrene will be continuously removed, which benefits selectivity. 

With normal interrupting devices the fault current would last for only a few cycles (maximum up to one or three seconds, depending upon the system design). This time is too short to allow heat dissipation from the conductor through radiation or convection. The total heat generated on a fault will thus be absorbed by the conductor itself. 

The convective wave cycle was described in 5.2.4 but its heat transfer properties not quantified. Critoph and Thorpe [22] and Thorpe [23] have measured the convective heat transfer coefficient between flowing gas and the grains within the bed. Preliminary results imply that the pressure drop through the bed can be expressed by a modified Ergun equation ... 

The fifth type of passive system is the natural convective loop, in which the collector is placed below the living space and the hot air that is created rises to provide heat where it is needed. This same principle is nsed in passive solar hot water heating systems known as thermosiphons. The storage tank is placed above the collector. Water is heated in the collector, becomes less dense, and rises (converts) into the storage tank. Colder water in the storage tank is displaced and moves down to the collector where it is heated to continue the cycle. 

Dry-heat processes kill microorganisms primarily through oxidation. The amount of moisture available to assist sterilization in dry-heat units varies considerably at different locations within the chamber and at different time intervals within the cycle. Also, the amount of heat available, its diffusion, and the environment at the spore/air interface all influence the microorganism kill rate. Consequently, cycles tend to be longer and hotter than would be expected from calculations to ensure that varying conditions do not invalidate a run. In general, convection dry-heat sterilization cycles are run above 160°C [37],... 

For liquid metals the superiority of nucleate boiling heat transfer coefficients over those for forced-convection liquid-phase heat transfer is not as great as for ordinary liquids, primarily because the liquid-phase coefficients for liquid metals are already high, and the bubble growth period for liquid metals is a relatively short fraction of the total ebullition cycle compared with that for ordinary fluids. In the case of liquid metals, the initial shape of the bubbles is hemispheric, and it becomes spherical before leaving the heating surface. This is because of very rapid... 

Dry air rising in the atmosphere has to expand as the pressure in the atmosphere decreases. This pV work decreases the temperature in a regular way, known as the adiabatic lapse rate, Td, which for the Earth is of order 9.8 Kkm-1. As the temperature decreases, condensable vapours begin to form and the work required for the expansion is used up in the latent heat of condensation of the vapour. In this case, the lapse rate for a condensable vapour, the saturated adiabatic lapse rate, is different. At a specific altitude the environmental lapse rate for a given parcel of air with a given humidity reaches a temperature that is the same as the saturated adiabatic lapse rate, when water condenses and clouds form Clouds in turn affect the albedo and the effective temperature of the planet. Convection of hot, wet (containing condensable vapour) air produces weather and precipitation. This initiates the water cycle in the atmosphere. Similar calculations may be performed for all gases, and cloud layers may be predicted in all atmospheres. 

Dry heat is used to sterihze and depyrogenate components and drug products. The definition of dry heat sterilization is 170 °C for at least 2 hours and a depyrogenation cycle at 250 °C for more than 30 minutes. Typical equipment includes tunnel sterilizers (force convection, infrared, fiame) and microwave sterilizers. An important aspect is the need to ensure air supply is filtered through HEPA filters. Biological indicators such as Bacillus subtilis can be used to gauge the performance of sterilization. 

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