Heat transfer at constant

FIGURE 6.12 A bomb calorimeter is used to measure heat transfers at constant volume. The sample in the central rigid container called the bomb is ignited electrically with a fuse wire. Once combustion has begun, energy released as heat spreads through the walls of the bomb into the water. The heat released is proportional to the temperature change of the entire assembly. 

Because heat transferred at constant volume can be identified with the change in internal energy, AU, we can combine Eq. 8 with C = q/AT and define the heat capacity at constant volume, Cv, as... 

We can use Eq. 1 to calculate the entropy change of the surroundings, provided that we assume that the surroundings are so large that their temperature and pressure remain constant. If the enthalpy change of the system is AH, then, for heat transfers at constant pressure, gSLlrr = —AH. We can now use Eq. 1 to write... 

J 8 Estimate the change in entropy of the surroundings due to heat transfer at constant pressure and temperature (Example 7.10). 

We have also seen that the heat transferred at constant pressure can be identified with the change in enthalpy, AH. It follows that the heat capacity at constant pressure, CP, is... 

Alternatively, a reaction might be carried out in an open flask or other apparatus that keeps the pressure constant and allows the volume of the system to change freely. In such a case, AU 0 and the energy change in the system might be due to both heat transfer and PV work. We indicate the heat transfer at constant pressure by the symbol qp ... 

Heat Transfer at Constant Volume Bomb Calorimeters... 

Most chemical reactions are done under constant (atmospheric) pressure conditions rather than at constant volume, so it is desirable to relate the heat transferred at constant pressure, q, to some state property analogous to the internal energy, U. 

Heat transfer at constant volume AU = q + w = q. Reactions to measure AU are performed in a constant volume calorimeter called a bomb calorimeter. 

Heat transfer at constant volume, qy (notation not used in the text)... 

Heat Transfers at Constant Pressure Definition of enthalpy, H... 

Irreversible deposition of a tracer from the flow onto a channel wall is analogous to heat transfer at constant wall temperature. The formulae can be applied to our problems straightforwardly, because there are no gradients of macroscopic concentrations and no temperature fields to change the properties of the bulk fluid. 

Enthalpy (FT) Measured in joules, H = E + pV the amount of energy in a system capable of doing work the sum of the internal energy of the system plus the product of its volume times the pressure on it. The change in enthalpy (AH) is equal to the heat transferred at constant pressure. 

For sensible heat transfer at constant specific heat, a heat balance on the process fluid gives... 

Convective heat transfer at constant Nusselt number... 

However, an increase in the heat transfer at constant pressure drop, which can be expressed as the ratio of the Nusselt number and the Poiseuille number Po, is not achieved for most channel geometries. Po is the ratio of the friction factor / according to Eq. (7.2) to Re. Drese and Hardt showed that diamond-shaped fins have the potential to increasing the Nu/Po ratio by a factor of more than 2, if the ratio of fin length to fin width is increased by a factor of 4, because recirculation zones are suppressed by the longer fins [9]. 

The special relationship between AS and the heat transferred in a reversible process is similar to the special relationship between AH and the heat transferred at constant pressure. (Section 5.3)... 

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