Hany A. Mohamed

Entropy Generation in Counter Flow Gas to Gas Heat Exchangers

Analysis of heat transfer and fluid flow thermodynamic irreversibilities is realized on an example of a counter flow double pipe heat exchanger utilizing turbulent air flow as a working fluid. During the process of mathematical model creation and for different working and constructing limitations, total thermodynamic irreversibility is studied. The present work proves that the irreversibility occurred due to unequal capacity flow rates (flow imbalance irreversibility). It is concluded that the heat exchanger should be operated at effectiveness, e, greater than 0.5 and the well operating conditions will be achieved when e approaches one where low irreversibility is expected. A new equation is adopted to express the entropy generation numbers for imbalanced heat exchangers of similar design with smallest deviation from the exact value. The results obtained from the new equation are compared with the exact values and with those obtained by Bejan (Bejan, A., 1997, Advanced Engineering Thermodynamics, Wiley, New York).

Repulsion-based model for contact angle saturation in electrowetting

We introduce a new model for contact angle saturation phenomenon in electrowetting on dielectric systems. This new model attributes contact angle saturation to repulsion between trapped charges on the cap and base surfaces of the droplet in the vicinity of the three-phase contact line, which prevents these surfaces from converging during contact angle reduction. This repulsion-based saturation is similar to repulsion between charges accumulated on the surfaces of conducting droplets which causes the well known Coulombic fission and Taylor cone formation phenomena. In our model, both the droplet and dielectric coating were treated as lossy dielectric media (i.e., having finite electrical conductivities and permittivities) contrary to the more common assumption of a perfectly conducting droplet and perfectly insulating dielectric. We used theoretical analysis and numerical simulations to find actual charge distribution on droplet surface, calculate repulsion energy, and minimize energy of the total system as a function of droplet contact angle. Resulting saturation curves were in good agreement with previously reported experimental results. We used this proposed model to predict effect of changing liquid properties, such as electrical conductivity, and system parameters, such as thickness of the dielectric layer, on the saturation angle, which also matched experimental results.

Effect of Rotation and Surface Roughness on Heat Transfer Rate to Flow through Vertical Cylinders in Steam Condensation Process

The enhancement in the rate of the heat transfer resulting from rotating smooth and rough vertical cylinders, of 1.28 and 21.75 μm average roughness, respectively, are experimentally studied. Experiments were carried out for cooling fluid Reynolds numbers from 3300 to 7800 with varying the rotational speed up to 280 rpm. Experimental runs at the stationary case showed an acceptable agreement with the theoretical values. The experimental Nusselt number values at various rotational speeds are correlated as functions of Reynolds, Weber, and Prandtl numbers for smooth and rough surfaces. The correlated equations were compared with the correlation obtained by another author. The results show that the enhancement of the heat transfer rate becomes more appreciable for low Reynolds numbers at high rotational speeds and for high Reynolds numbers at low rotational speeds. The rotation causes an enhancement in the overall heat transfer coefficient of 89% at Re = 7800, We = 1084, and Pr = 1.48 for smooth surface and of ∼13.7% at Re=4700, We=4891, and Pr=1.696 for rough surface. Also, the enhancement in the heat transfer rates utilizing rotary surface becomes more pronounced for the smooth surface compared with the rough one, therefore the choice of the heat transfer surface is very important. The present work shows a reduction in the heat transfer rate below its peak value depending on the type of the heat transfer surface. It is shown that the enhancement in the heat transfer, i.e., enhancement in the Nusselt number; depends on the Weber number value and the surface type while the Nusselt number value mainly depends on the Reynolds and Prandtl numbers. Correlated equation have been developed to represent the Nusselt number values as functions of the Weber and Reynolds numbers within the stated ranges of the parameters.

Optimum performance comparative study of combined gas turbine partial oxidation cycle and reheat cycle

The present paper studies the general characteristics and evaluate the optimum performance of 2-stage POGT cycle augmented by a recuperation process. A comparative study of the performance of POGT cycle with that of a reheat cycle (RHGT) is discussed. The ranges of values of the controlling parameters of both cycles that give optimum performance (e.g. first law and second law efficiencies and net work output /spl eta//sub I/, /spl eta//sub II/, and w/sub net/ respectively) are determined and fitted into functional correlation equations. The effects of irreversibilities of the different composing units of the two cycles are accounted for in the study. The present findings can form a very important basis for a complete phenomenological design of POGT and RHGT cycles to achieve optimum performance. The study shows that for POGT cycle, at constant /spl theta/, the effect of x on q is very small compared to its effect on /spl eta//sub I/ and /spl eta//sub II/, reaching their maximum values at about x=2.8. Mass ratio in has a constant decreasing effect on all cycle performance characteristics. Higher values of /spl theta/ result in higher values of all cycle characteristics. For the RHGT cycle, x greatly effects all performance characteristics and has some limiting values at which w/sub net/=/spl eta//sub I/=0. Variation trends of cycle characteristics are similar in regards to the effect of /spl theta//spl mu/ with their optimum values tends to lie between values of x of 1.6 and 2.6. Higher values of x decrease cycle performance. The comparative study shows that /spl eta//sub I/ for the RHGT cycle always exceeds that of the POGT cycle only at low values of x. Beyond these values of x, the POGT is better than the RHGT. The correlation equations relate optimum values of both cycle characteristics to the controlling parameters of each cycle. Irreversibilities of different units of each cycle have been accounted for in these equations.

Comparative study of steam injection effects on operation of gas turbine cycles

In the present work, gas turbine cycles are modified with steam injection between the combustion chamber exit and the gas turbine inlet. Heat recovery steam generators, utilising the exhaust gases, provide these cycles with the injected steam at saturated vapour. The irreversibility of the different composing units of the cycles and the variation of gas properties owing to steam injection as well as changes in the interrelation of component performance parameters are taken into account. The isentropic temperature ratio and maximum to minimum cycle temperature ratio are varied over some ranges that slightly exceed their practically acceptable bounds in order to comprehensively investigate their effects on cycle characteristics. At the chosen values of the operating parameters, the enhancement achieved in the overall efficiency for the simple, reheat (with steam injection at high and low pressures) and partial oxidation (with steam injection at high and low pressures) gas turbine cycles are of about 20∼30%, 120∼200%, 10∼12%, 120∼260%, 20% respectively. The present modified cycles technique can be considered among the possible ways to improve the performance of gas turbine cycles-based power plants at feasible costs.