Mixed Heat Transfer Research Articles

An Experimental and Analytical Investigation of the Gas Exchange Process in a Multi-Cylinder Pressure-Charged Two-Stroke Engine

The results are given of a comprehensive investigation of the pressure and temperature diagrams in a multi-cylinder two-stroke engine during the gas-exchange process. A six-cylinder turbocharged two-stroke diesel engine was fully instrumented to record transient pressures in three cylinders, in the scavenge belt, and in a number of positions in a specially designed exhaust pipe. Transient-temperature records were taken in the exhaust pipe. Steady-flow tests were carried out on the inlet ports, the exhaust valves, and the exhaust pipe. The steady-flow data were then used in a computer programme (see references (9) (12)‡) to predict the pressure in the cylinder and the pressures and temperatures in the exhaust pipe. Discounting some measured cylinder-pressure records, which were suspect, the investigation showed good agreement between the predicted and measured pressures in the cylinders and exhaust pipe. The analysis of the temperature records showed there was evidence of mixing and longitudinal heat transfer in the exhaust pipe; however, the mean computed temperature in the exhaust pipe at the nozzle end agreed with the measured temperature. Comparison of the measured and computed mass flows at the nozzle end of the pipe using the theoretical diagrams gave good agreement. It was concluded that the computer programme was a satisfactory method for calculating the pressures in the cylinder and exhaust pipe during the gas-exchange process.

Thermal energy from the earth's crust

Abstract The characteristics of the Jarger and hotter natural hydrothermal systems are discussed, and several different models of heat source and heat-transfer mechanism reviewed. It is shown that, with models depending on mixed conductive-convective heat transfer from a magma chamber, the observed heat flows reauire either a contact area of several hundred square kilometres for the conductive transfer link, or very small separations between a chamber of circulating magma and the convective hot water system. None of the models is entirely satisfactory, and the possibility of heat transfer by magmatic steam is reconsidered in the light of the new isotope evidence, which indicates that most of the water discharged is of surface origin. It is suggested that it would be possible for this surface water to penetrate to and become absorbed by heated rock or magma at great depths and pressures, and for it to be ejected from the rock by subsequent heating, or to escape under reduced pressure to provide a very efficient high-temperature heat-transfer medium that would mix with ground water near the surface. The power potential of the various models is estimated by calculating the mechanical energy that could be obtained from an ideal heat engine taking heat from the rock or water and discharging the heat into a sink at a fixed temperature of 30°C. The results are plotted as graphs of available power per unit mass flow against temperature for saturated steam and water up to the critical temperature, and energy content per unit volume against temperature for steam, water, and rock. In nearly all cases, most of the stored energy is in the rock. The total energy content of the conductive and convective zones of the models discussed is estimated from their probable dimensions, and is found to be of the order of 106 to 108 megawatt years for the larger models. At the power drawoff rates of 250 to 350 megawatts involved in the Wairakei and Larderello power projects, these quantities of energy, if effectively drawn upon, would last at least several thousand years, and even the smallest probable model would supply these stations for some hundreds of years. If any of these models is in fact applicable to the areas concerned, it is unlikely that anv appreciable impression on the storage will be made over several decades; and neither the Wairakei nor the Larderello areas are yet showing any sign of exhaustion of their heat supply, though there are indications that the water supply may become a limiting factor. Eventually, both these and other hydrothermal power projects may gain great advantages from control of the water supply and circulation by injection of recharge water.

Axial Flow Operations with Longitudinal Dispersion and Chemical Reaction

Methods of predicting the extent of chemical reaction or mass transfer are presented for onedimensionalaxial flow systems accompanying longitudinal dispersion, chemical reaction and othertransport phenomena. Four typical cases are treated as follows.Case 1 Two-phase mass-transfer operation with fast irreversible second-order reaction:Basic differential equation, Eq.(4), for the column operation are reduced to those for simplemass-transfer case by defining the nondimensional terms included in Eq.(5) as Eq.(6). Previoustheoretical analysis is indicated entirely applicable to this new situation by defining those terms inthis manner.Case 2 Two-phase mass-transfer operation with slow first-order irreversible reaction:With negligible reactant concentration in the liquid bulk of phase Y this case is indicatedequivalent to that of one-dimensional homogeneous phase reactor accompanying simple first-orderreaction. When the reactant concentration in phase Y has a finite value, analytical solution of thebasic rate equation is complicated in its form and not convenient for design purpose (see appendix).Case 3 Homogeneous-phase nonisothermal chemical reactor of back-flow model type:Chemical reactors of the type of back-flow model, which assumes multi-cell type column withinterstage mixing of fluid between adjacent perfectly mixed cells, is treated in this section and thenext following.For homogeneous case the operation is shown in Fig. 2. Eq.(11)-(16) are obtained for heat andmass balances, and used to calculate the values at the 1st stage from the values of the N-th stage, which must be fixed at first. Sample calculations for the case of first order reaction are shown in Fig. 3 to 6; Fig. 3 and 4 show the effect of interstage mixing, reaction rate, and wall heat transfer; Fig. 5, 6 show the effect of the number of division. These are calculated by a digital computerHIPAC-101. Solutions by this method are exact, when interstage mixing really exists, and are goodapproximate solution for the diffusion model with enough higher number of division, because thedifferential equations of the diffusion model (Eq.(17), (18)) can be written in a finite differenceform same as Eq.(11)-(16). P-B has a relation expressed by Eq.(19) for heat and mass.Case 4 Two-phase nonisothermal cell-model reactor with interstage mixing:Operation is shown in Fig. 7. When the rate processes taking place in the dispersed phase isfirst order, this phase can be regarded as the second continuous phase under some restriction. Therefore the basic equations are Eq.(22)-(30), where interstage mixing flow of phase Y (dispersed)is neglected, since usually the hold-up of this phase is rather small. As an example, countercurrent nitration of toluene by mixed acids is treated and shown in Fig. 8. In this case, 6 stages are enough to convert 90% of toluene to mononitrotoluene. These methods are enoughuseful for design purposes in such complicated cases.

Convective Heat Transfer for Mixed, Free, and Forced Flow Through Tubes

Abstract This paper presents current knowledge on heat transfer caused by simultaneously occurring free and forced convection in tubes. It discusses the results of experiments that have been conducted by the NACA on mixed, free, and forced-convection heat transfer in turbulent flow through a vertical tube with a length-to-diameter ratio of 5. In these studies, forced air flow was directed parallel or opposite to the flow direction which free-convection flow alone would have under the same temperature conditions. Results of experiments made by other investigators, when analyzed in the same way as the NACA experiments, make it possible to generalize and extend the heat-transfer relations established. A calculation by R. C. Martinelli and L. M. K. Boelter for laminar mixed, free, and forced flow in parallel direction, and its extension to flow in opposite direction furnish information on heat transfer in the laminar-flow range.