Enthalpy flux

The enthalpy flux vector N per unit total cross section in the... 

Then in terms of h and the enthalpy flux vector the enthalpy... 

Equations (12.13) and (12.14) may be approximated by rather simple equations in most conditions of physical interest. This is possible because of the relatively large value of the thermal conductivity of the solid matrix, which has two important consequences. First, the conductive enthalpy flux, represented by the second term on the left hand side of... 

Fig. 6.2 shows a simplified diagram of the basic STIG plant with steam injection S per unit air flow into the combustion chamber the state points are numbered. Lloyd 2 presented a simple analysis for such a STIG plant based on heat input, work output and heat rejected (as though it were a closed cycle air and water/steam plant, with external heat supplied instead of combustion and the exhaust steam and air restored to their entry conditions by heat rejection). His analysis is adapted here to deal with an open cycle plant with a fuel input/to the combustion chamber per unit air flow, at ambient temperature To, i.e. a fuel enthalpy flux of/7i,o. For the combustion chamber, we may write... 

Again we analyse the open cycle version of this plant, but with a fuel input/ (per unit air flow) at ambient temperature Tq, i.e. a fuel enthalpy flux of/ /rfo- For the combustion chamber we may now write... 

On expanding the enthalpy-flux term on the right-hand side as... 

The mixing length theory relates the convective (enthalpy) flux F to the envelope structure quite well (Chan and Sofia 1987). For... 

An example of the distributions of fluxes versus depth, ln(pressure). The diffusive flux (dotted curve) is very small except near boundaries. The flux of kinetic energy (dashed curve) is downward and its magnitude reaches about 50% of the total flux (thick solid curve). This makes the enthalpy flux (thin solid curve) nonuniform. 

Temperature variation owing to enthalpy flux associated with permeation f 0 on reaction side... 

Each of the reactions (r) contributing to the overall metabolic activity has a characteristic molar enthalpy, A,//b (J moL1), where subscript B indicates that any given reaction stoichiometry must be divided by vB (v = stoichiometric number) to give a stoichiometric form of unity (IVbI = 1). Each is calculated from the balanced reaction stoichiometry and enthalpy of formation. If this value is multiplied by the measured chemical reaction flux, JB (mol s 1 m-1), then the reaction enthalpy flux, JhO mo -3), is obtained,... 

This forms the basis of constructing an enthalpy budget in which the total enthalpy flux is compared with the scalar heat flux, 7q(W m-3), obtained from dividing heat flow by size (volume or mass) of the living matter. If account is made of all the reactions and side reactions in metabolism, the ratio of heat flux to enthalpy flux, the so-called energy recovery ( Yq/H = Jq/Jh) will equal 1. If it is more than 1, then the chemical analysis has failed fully to account for heat flux and if it is less than 1, then there are undetected endothermic reactions. Account for all reactions may seem a formidable task, but it should be borne in mind that anabolic processes dissipate insignificant amounts of heat compared with those of catabolism and that ATP production and utilization are balanced in cells at steady-state. Catabolism is generally limited to a relatively few well-known pathways with established overall molar enthalpies. So, as will be seen later, the task is by no means mission impossible. ... 

The enthalpy flux caused by mass transport from the gas channels to the electrodes is considered negligible. 

The mean heat transfer rate up to the value of Z being considered can be obtained by integration of he local wall heat transfer rate or by noting that the total heat transfer rate up to the point considered will be equal lo the increase in the enthalpy flux between the inlet and the c value considered, i.e., that ... 

Enthalpy flux relative to that + at the saturation temperature for the film at the section considered... 

Equation (24) states that the sum of the enthalpy flux and the ordered kinetic-energy flux may change only because of heat flux ( 2 3 ) viscous dissipation [ 2 PsiP Pi i) ] Equation (24) also illustrates that the enthalpy is of more importance than the internal energy in steady-flow problems the additional term p/p, which appears when equation (24) is written in terms of u, may be explained as displacement work. ... 

This sum often is called the stagnation enthalpy flux and includes thermal enthalpy, chemical enthalpy, and kinetic energy. 

In this form of equation (24), the effect of diffusion on the enthalpy flux is included implicitly in the first term, and the only term from the heat-flux vector that remains explicitly is that of heat conduction. The above equations serve to eliminate completely the diffusion velocities from the governing equations, replacing them by the flux fractions. The fact that equations (31) and (33) appear to be less complicated than equations (22) (with dYJdt = 0) and (24) often justifies this transformation. 

Negligible enthalpy flux due to fluid species diffusion. 

Similarly, an increment of 8T in the reactor temperature will produce an increment in the enthalpy flux in the product stream of qCp8T = 8Qf. It will also increase the rate of cooling by dQ ldT) 8T and the rate of cooling due to heating the incoming feed by qCp 8T, Hence the total heat... 

Corresponding to Equation 9.11 is a modified equation for femperafure fhaf is based on the enthalpy flux ... 

Similar equations can be written for the other components and for the enthalpy flux. An equation for the pressure drop is also required. The boundary conditions are of the Danckwerts type if the system is closed and are those of Example 9.2 for an open system. The shooting method of solution can be used for the closed system. The open system is more easily solved using the method of false transients described in Chapter 16. 

The tninimum ratio of the enthalpy flux to the flux of products is equal to... 

It follows from (6.235) that the total flux of energy at the drop surface is equal to the sum of enthalpy fluxes of evaporating components and the energy flux transferred by the substance. If mass exchange is absent, then d 0 and d/ e 1) —> 1. If we decide to account for the mass exchange, then d 0 and... 

In Eq. 4.34, the effect of enthalpy flux associated to hydrogen outgoing from the reaction zone through the membranes is neglected. 

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