Heat convection definition

In both convective heat transfer definitions it is presumed that the solid surface is warmer than the fluid so that heat is being transferred from the solid to the fluid. Equations (5.125) and (5.127) are sometimes called Newton s law of cooling but are merely the defining equation for the hk parameters [102, 60, 15]. 

To simplify the effects of radiation and convection on dry heat transfer, the concept of operative temperature is often used. By definition operative temperature is the temperature of a uniform environment (= MRT) that has the same total dry heat loss (convection + radiation) as the actual environment where MRT. 

The transfer of heat by radiation in general can be said to occur simultaneously with heat transfer by convection and conduction. Transfer by radiation tends to become more important than that by the other two mechanisms as the temperature increases. It is useful to gain an appreciation of the basic definitions of the energy flux terms, the surface property terms and their relationships while discussing radiative heat transfer. With this objective, reference may be made to Table 3.4 in which these are presented. 

The lack of hydrodynamic definition was recognized by Eucken (E7), who considered convective diffusion transverse to a parallel flow, and obtained an expression analogous to the Leveque equation of heat transfer (L5b, B4c, p. 404). Experiments with Couette flow between a rotating inner cylinder and a stationary outer cylinder did not confirm his predictions (see also Section VI,D). At very low rotation rates laminar flow is stable, and does not contribute to the diffusion process since there is no velocity component in the radial direction. At higher rotation rates, secondary flow patterns form (Taylor vortices), and finally the flow becomes turbulent. Neither of the two flow regimes satisfies the conditions of the Leveque equation. 

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. 

The problem of burn-out prediction is a difficult one, and one on which a great deal of experimental work is being carried out, particularly in connection with nuclear-reactor development. Much of the earlier literature is rather confused, due to the fact that the mechanics of the burn-out were not carefully defined. Silvestri (S8) has discussed the definitions applicable to burn-out heat flux. It appears possible to define two distinctly different kinds of burn-out, one due to a transition from nucleate to film boiling, and one occurring at the liquid deficient point of the forced-convection region. The present discussion treats only the latter type of burn-out fluxes. The burn-out point in this instance is usually determined by the sudden rise in wall temperature and the corresponding drop in heat flux and heat-transfer coefficient which occur at high qualities. 

TEMPERATURE. The thermal state of a body, considered, with reference to its ability to communicate heat to other bodies (J. C. Maxwell). There is a distinction between temperature and heat, as is evidenced by Helmholtz s definition of heat, [energy that is transferred from one body to another by a thermal process), whereby a thermal process is meant radiation, conduction, and/or convection. 

Problem definition requires specification of the initial state of the system and boundary conditions, which are mathematical constraints describing the physical situation at the boundaries. These may be thermal energy, momentum, or other types of restrictions at the geometric boundaries. The system is determined when one boundary condition is known for each first partial derivative, two boundary conditions for each second partial derivative, and so on. In a plate heated from ambient temperature to 1200°F, the temperature distribution in the plate is determined by the heat equation 8T/dt = a V2T. The initial condition is T = 60°F at / = 0, all over the plate. The boundary conditions indicate how heat is applied to the plate at the various edges y = 0, 0[Pg.86]

The heat transfer rate, q, is taken as positive in the direction wall-to-fluid so that it will have the same sign as(Tw -T/) and h will always, therefore, be positive. A number of names have been applied to h including convective heat transfer coefficient , heat transfer coefficient , film coefficient , film conductance , and unit thermal convective conductance . The heat transfer coefficient, h, has the units W/m2-K or, since its definition only involves temperature differences, W/m2oC, in the SI system of units. In the imperial system of units, h has the units Btu/ft2-hr-°F. 

An adiabatic refractory surface of area Ar and emissivity er, for which Qr = 0, proves quite important in practice. A nearly radiatively adiabatic refractory surface occurs when differences between internal conduction and convection and external heat losses through the refractory wall are small compared with the magnitude of the incident and leaving radiation fluxes. For any surface zone, the radiant flux is given by Q = A(W - H) = tA(E - H) and Q = eA/p( - W) (if p 0). These equations then lead to the result that if Qr = 0,Er = Hr = Wrfor all 0 < er< 1. Sufficient conditions for modeling an adiabatic refractory zone are thus either to put , = 0 or to specify directly that Q, = 0 with , 0. If er = 0, SrSj = 0 for all 1 < j < M which leads directly by definition to Qr = 0. For er = 0, the refractory emissive power Er never enters the zoning calculations. For the special case of 0 and Mr = 1, a sin-... 

Using the definition of 7, the convection heat transfer coefficient becomes... 

This indicates that one part of the heat does not flow directly from wall to fluid. A longitudinal heat flow exists and Agostini [30] and Commenge [31] give a rule to estimate whether or not the conditions required for a purely transversal heat flow are fulfilled. They define a Biot number which allows us to compare the convective heat flow and the conductive longitudinal heat flow. The former gives the definition... 

Agostini shows that for Bi > 3 the convective effects are prominent and for Bii < 0.3 the longitudinal heat flux produces an effect on the temperature profiles. The definition given by Commenge was calculated for counter-current heat exchangers and leads to different valnes. Evalnating these nnmbers would be useful in ensuring the heat flux is purely transversal. 

Conduction is the most widely understood mechanism of heat transfer and the main method in solids. The flow of heat depends upon the transfer of vibrational energy from one molecule to another and, in the case of metals, the movement of free electrons. Radiation is rare in solids but examples are found among glasses and plastics. Convection by definition, is not possible under these conditions. Conduction in the bulk of fluids is normally overshadowed by convection, but it assumes great importance at fluid boundaries. 

Due to the Langrangian formulation applied to the solid phase, the use of an effective thermal conductivity as usually applied to porous media is not necessary. In a packed bed heat is transported between solid particles by radiation and conduction. For materials with low thermal conductivity, such as wood, conduction contributes only to a minor extent to the overall heat transport. Furthermore, heat transfer due to convection between the primary air flow through the porous bed and the solid has to be taken into account. Heal transfer due to radiation and conduction between the particles is modelled by the exchange of heat between a particle and its neighbours. The definition of the neighbours depends on the assembly of the particles on the flow field mesh. 

In this paper, we now report measurements of heat transfer coefficients for three systems at a variety of compositions near their lower consolute points. The first two, n-pentane--CO2 and n-decane--C02 are supercritical. The third is a liquid--liquid mixture, triethylamine (TEA)--H20, at atmospheric pressure. It seems to be quite analogous and exhibits similar behavior. All measurements were made using an electrically heated, horizontal copper cylinder in free convection. An attempt to interpret the results is given based on a scale analysis. This leads us to the conclusion that no attempt at modeling the observed condensation behavior will be possible without taking into account the possibility of interfacial tension-driven flows. However, other factors, which have so far eluded definition, appear to be involved. 

In practice, for any practical psychrometer or wetted droplet or particle, there is significant extra heat transfer from radiation. For an Ass-mann psychrometer at near-ambient conditions, this is approximately 10 percent. This means that any measured real value of T b is slightly higher than the "pure convective value in the definition. It is often more convenient to obtain wet-bulb conditions from adiabatic saturation conditions (which are much easier to calculate) by the following formula ... 

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 definitions of the heat and mass transfer fluxes are thus merely based on empirical arguments, so in the literature there are given more than one way to interpret the transfer coefficients [15, 139]. Basically, the transfer coefficients are either treated as an alternative model to the fundamental diffusion models (i.e., the Fourier s and Pick s laws) or the transfer coefficients are taking both diffusive and convective mechanisms into account through empirical parameterizations. However, in reaction engineering practice the distingtion between these approaches is rather blurred so it is not always clear which of the fundamental transport processes that are actually implemented. 

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