Noam Lior

Jet Impingement Heat Transfer: Physics, Correlations, and Numerical Modeling

Publisher Summary This chapter presents a discussion on jet impingement heat transfer. The chapter describes the applications and physics of the flow and heat transfer phenomena, available empirical correlations and values they predict, and numerical simulation techniques and results of impinging jet devices for heat transfer. The relative strengths and drawbacks of the Reynolds stress model, algebraic stress models, shear stress transport, and v 2 f turbulence models for impinging jet flow and heat transfer are compared in the chapter. The chapter provides select model equations as well as quantitative assessments of model errors and judgments of model suitability. The review of recent impinging jet research publications identified a series of engineering research tasks that are important for improving the design and resulting performance of impinging jets: (1) clearly resolve the physical mechanisms by which multiple peaks occur in the transfer coefficient profiles, and clarify which mechanism(s) dominate in various geometries and Reynolds number regimes, (2) develop a turbulence model, and associated wall treatment if necessary, that reliably and efficiently provides time-averaged transfer coefficients, (3) develop alternate nozzle and installation geometries that provide higher efficiency, meaning improved Nu profiles at either a set flow or set blower power, and (4) further explore the effects of jet interference in jet array geometries, both experimentally and numerically. This includes improved design of exit pathways for spent flow in array installations.

Sources of Combustion Irreversibility

Abstract Approximately 1/3 of the useful energy of the fuel is destroyed during the combustion process used in electrical power generation. This study is an attempt to clarify and categorize the reasons for the exergy destruction taking place in combustion processes. The entropy production is separated into three subprocesses: (1) combined diffusion/fuel oxidation, (2) “internal thermal energy exchange” (heat transfer), and (3) the product constituent mixing process. Four plausible process paths are proposed and analyzed. The analyses are performed for two fuels: hydrogen and methane. The results disclose that the majority (about 3/4) of the exergy destruction occurs during the internal thermal energy exchange. The fuel oxidation, by itself, is relatively efficient, having an exergetic efficiency of typically 94% to 97%.

Impingement Heat Transfer: Correlations and Numerical Modeling

Uses of impinging jet devices for heat transfer are described, with a focus on cooling applications within turbine systems. Numerical simulation techniques and results are described, and the relative strengths and drawbacks of the κ-e, κ-ω, Reynolds stress model, algebraic stress models, shear stress transport, and ν2f turbulence models for impinging jet flow and heat transfer are compared.

The theory and practice of energy saving in the chemical industry: some methods for reducing thermodynamic irreversibility in chemical technology processes

The causes of thermodynamic irreversibility in chemical reactions and other industrial chemical processes (in particular absorption, stripping, and heat transfer) and ways of reducing energy consumption have been examined. Some thermodynamic principles based on the Second Law of thermodynamics such as the so called “counteraction principle,” “driving force method,” “quasi-static method,” and the result some of practical methods for energy saving design are discussed. It is demonstrated that the possibilities for reducing energy consumption are substantially higher than often seems possible. The correctness and practical effectiveness of the above methods have been confirmed by many commercial examples, for instance the lead author was able to reduce heat consumption in more than 20 commercial CO2 removal installations by changes in networks only, without changing the absorbent. The heat consumption was reduced to about 1/2 to 1/3 of that used with conventional flow sheets.

Numerical calculations of laminar and turbulent natural convection in water in rectangular channels heated and cooled isothermally on the opposing vertical walls

Abstract Natural convection was computed by finite-difference methods using a laminar model for 2 (wide) × 1 and 1 × 1 enclosures for Ra from 106 to 109 and Pr = 5.12 and 9.17, and a k-ϵ turbulent model for a square enclosure for Ra from 1010 to 1011 and Pr = 6.7. The average Nusselt numbers agree well with the correlating equation of Churchill for experimental and computed values. The computed velocity profile along the heated wall is in reasonable agreement with prior experimental values except for the thin boundary layer along the lower part of the wall where a finer grid size than was computationally feasible appears to be necessary. A detailed sensitivity test for constants of the k-ϵ model was also carried out. The velocity profile at the middle height and the average Nusselt number was in even better agreement with the experimental results when the turbulent Prandtl number was increased to four and the constant c1 was decreased by 10%. A more refined turbulent model and finer grid divisions appear to be desirable, particularly for larger Ra.

Development of a Novel Combined Absorption Cycle for Power Generation and Refrigeration

Cogeneration can improve energy utilization efficiency significantly. In this paper a new ammonia-water system is proposed for the cogeneration of refrigeration and power. The plant operates in a parallel combined cycle mode with an ammonia-water Rankine cycle and an ammonia refrigeration cycle, interconnected by absorption, separation, and heat transfer processes. The performance was evaluated by both energy and exergy efficiencies, with the latter providing good guidance for system improvement. The influences of the key parameters, which include the basic working solution concentration, the cooling water temperature, and the Rankine cycle turbine inlet parameters on the cycle performance, have been investigated. It is found that the cycle has a good thermal performance, with energy and exergy efficiencies of 27.7% and 55.7%, respectively, for the base-case studied (having a maximum cycle temperature of 450 degrees C). Comparison with the conventional separate generation of power and refrigeration having the same outputs shows that the energy consumption of the cogeneration cycle is markedly lower A brief review of desirable properties of fluid pairs for such cogeneration cycles was made, and detailed studies for finding new fluid pairs and the impact of their properties on cogeneration system performance are absent and are very recommended.

Thermal performance and exergy analysis of a thermal vapor compression desalination system

Abstract A thermodynamic analysis based on the first and second laws is conducted to evaluate the performance of a thermal vapor compression (TVC) desalination system. The performance of the analytical model is compared with operational data obtained from tests performed on a four-effect, low temperature TVC desalination system with performance ratios of 6.5–6.8, located in the U.A.E. The effect of the process variables on the plants performance ratios is investigated. The exergy losses due to irreversibilities in different subsystems of the TVC system are evaluated and compared with those of the conventional multi-effect boiling (MEB) and mechanical vapor compression (MVC) desalination systems. The TVC system yields the least exergy destruction among the three systems. Subsystem exergy analysis shows that most of the exergy destruction in the TVC system occurs in the first effect and in the thermo-compressor. Overall exergy losses can be significantly reduced by increasing the number of effects and the thermo-compressor entrainment ratio, and by decreasing the top brine and heating steam temperatures.

COMBINING FUEL CELLS WITH FUEL-FIRED POWER PLANTS FOR IMPROVED EXERGY EFFICIENCY

The thermodynamic advantages of fuel-cell systems are studied to: 1.(i) evaluate the increase in plant exergy efficiency when incorporating fuel-cell units into electrical power generating stations and2.(ii) identify and discuss their effect on the components of such plant systems. Topping conventional Rankine cycle power plants with a range of commercial fuel cells is shown to increase the exergetic efficiency of the plant by up to 49%, raising that efficiency from the value of 41.5% for the conventional power plant without fuel cells to about 62% for the fuel-cell-topped power plant. This improvement stems primarily from the improved exergetic efficiency of fuel oxidation in these proposed topping power plants, as contrasted with the highly dissipative combustion process in conventional fuel-fired ones.

The Component Equations of Energy and Exergy

Energy conversion processes inherently have associated irreversibility. A better understanding of energy conversion will motivate intuition to create new energy-conversion and energy-utilization technology. In the present article, such understanding is further enhanced by decomposing the equations of energy and exergy (availability, available energy, useful energy) to reveal the reversible and irreversible parts of energy transformations. New definitions of thermal, strain, chemical, mechanical and thermochemical forms of energy/exergy are justified and expressions for these properties and their changes are rigorously developed. In the resulting equations, terms appear which explicitly reveal the interconversions between the different forms of energy/exergy, including the breakdown into reversible and irreversible conversions. The equations are valid for chemically reacting or non-reacting inelastic fluids, with or without diffusion.

Thoughts about future power generation systems and the role of exergy analysis in their development

Abstract In face of the likely doubling of the world population and perhaps tripling of the power demand over the next 50 years, this paper (1) presents some thoughts on the possible ways to meet the power demands under the constraints of increased population and land use while holding the environmental impact to a tolerable one, and (2) outlines the ways exergy analysis may be effectively used in the conception and development of such processes. To effectively develop the innovative power generation systems needed in the 21st century, irreversibility and exergy analysis should be much more focused on the intrinsic process details.