Heat loss coefficients for box-type solar cookers

The top and overall heat loss coefficients for the entire feasible operating range of box-type solar cookers are evaluated experimentally and presented in a graphical form as a function of the difference between mean plate temperature and ambient temperature with wind velocity and number of glass covers as parameters.

The range of plate temperatures considered is from 50°C to 180°C. While the wind velocity is varied from 0 to 3.33 m/s, and the number of glass covers considered are from 1 to 4.

Based on these experimental results, a correlation for the determination of top loss coefficient in terms of optical properties of cooker, the spacing between glass cover and absorber plate, wind velocity and number of glass covers, is derived and presented in the paper.     

The law of stable equilibrium and the entropy-based boiling curve for flow boiling

Convective flow boiling in sub-cooled fluids is recognized as one of the few means of accommodating very high heat fluxes. There are many available correlations for predicting the inner wall temperature of the heated duct in the several regimes of the empirical Nukiyama boiling curve, although unfortunately there is no physical fundamentals of such curve. Recently, the author has shown that the classical entropy balance could contain key information about boiling heat transfer. So, it was found that the average thermal gap in the heated channel (the inner wall temperature minus the average temperature of the coolant fluid) was strongly correlated with the efficiency of a theoretical reversible engine placed in this thermal gap. From this new correlation, a new boiling curve plotting the wall temperature versus the average fluid temperature was derived and successfully checked against low- and high-pressure water data. This curve suggested a new and simple definition of the critical heat flux (CHF) namely, the value of the coolant average temperature at the maximum. In this work, after briefly reviewing the entropy balance of a non-equilibrium boiling flow and its relationship with the thermodynamic average temperature and the law of stable equilibrium (LSE), the possibilities of the new approach for the design of flow boiling cooling systems are highlighted. Finally, the strong correlation found between the reversible engine efficiency and the thermal driving force is verified again, now with high-pressure refrigerant 22 (R-22) data.     

An asymptotic approach to compressible boundary-layer flow

A regular perturbation approach is applied to take into account variable property effects. Of special interest is the effect of the pressure-dependent density. In the framework of an asymptotic theory, compressibility effects are considered as variable property effects. By means of this asymptotic theory the deviation of skin friction and heat transfer results from their incompressible, isothermal values are determined for laminar Falkner-Skan boundary layers. As far as laminar flow is concerned there is no need for any empirical information.     

Error of Darcy's law for serpentine flow fields: An analytical approach

Serpentine flow fields and other flow fields with partial under-land cross-flow are commonly used in various energy devices, such as proton exchange membrane (PEM) fuel cells and redox flow batteries, due to their higher mass transfer rate to reaction sites and better product removal capability. Accurately predicting the under-land cross-flow rate and pressure drop in such flow fields is crucial in flow field design optimizations. Darcy's law is the most commonly used model in predicting the under-land cross-flow and pressure drop in such flow fields. However, since the Darcy's law neglects inertial effect, its validity in different designs and operating conditions needs to be carefully studied. In this work, mathematical models for a serpentine flow field are developed based on both the Darcy's law and a modified Darcy's law that includes the inertial effect. Both models are solved and analytical solutions are obtained. The predicted pressure drops and under-land cross-flow rates from the two models are compared with experimental data and the results show that under some conditions, both the Darcy's law and the modified Darcy's law can predict pressure drop and under-land cross-flow rate reasonably well. However, under other conditions the Darcy's law can result in significantly large errors in predicting both pressure drop and under-land cross-flow rates. Further studies provide the variations of errors from the Darcy's law with different parameters, including channel length, gas diffusion layer (GDL) thickness, land width, inlet flow rate, GDL permeability and GDL inertial coefficient.     

An investigation on instantaneous local heat transfer coefficients in high-temperature fluidized beds—I. Experimental results

Experimental results are presented and analysed for instantaneous and time-averaged local surface temperatures and heat transfer coefficients around a water-cooled horizontal tube of 40 mm in outer diameter immersed in a 230 × 160 mm cross section high temperature fluidized bed. Such measurements are made using a specially-designed thin-film thermocouple probe. Silica sands with mean diameter of 0.662, 1.103 and 1.815 mm are used as bed materials. Measurements are taken over a wide temperature range from 270 to 1028°C.     

Thermal loss coefficients of low-density silica aerogel tiles

The thermal losses of a monolithic low-density SiO₂ aerogel tile (thickness d = 9 mm, density = 80 kg/m³) have been determined in our evacuable guarded hot plate system LOLA II. The measurements were performed for two values of boundary emissivities ( ≈ 0.05 and 0.9) under variation of temperature and of gas pressure for several types of gases. From our results we conclude that low-density silica aerogel tiles about 10 mm thickness provide excellent thermal insulation with overall loss coefficients k 0.7 W/(m² · K) at ambient temperatures (10°C or 283 K) and pressures 10 mbar. Their use in window systems and covers for passive use of solar energy thus is extremely promising. From the change of thermal losses with gas pressure, a mean free path within the (evacuated) skeleton of about 120 nm can be derived.     

A method of measuring windmill torque loss coefficients

The aim of this work is to investigate a method of measuring windmill torque loss coefficients of a windmill power system. We discussed the mathematical windmill model and defined the windmill torque loss coefficients. An estimating technique based on Kalman filtering was developed for decision of the value of these coefficients. We manifested the valdity of this method by the estimated results from both the simulated data and wind-tunnel experimental data.     

Solar heat gain factors and heat loss coefficients for passive heating concepts

The concept of solar heat gain factor has been introduced for calculating the net energy gain of passive heating elements and other components of a building as a result of incident solar radiation. For passive heating concepts (namely, the direct gain, mass wall, water wall, Trombe wall, and solarium), exact analytical expressions have been obtained for the solar heat gain factors and the corresponding overall heat loss coefficients. These will allow a building designer to calculate immediately the overall heat gain/loss in a building. Numerical calculations have been done for typical values of solar radiation and ambient temperature of typical climatic conditions in India. The method has been compared with the other methods reported in the literature so far. A good comparison is found between the earlier methods and the method of using solar gain factors and the corresponding heat transfer values.     

An analytical approach for a Hiemenz flow in a porous medium with heat exchange

In this work, a Hiemenz flow established inside a semi-infinite low porosity medium with heat exchange is analyzed analytically. Heat is supplied to the system through two different wall conditions: a constant wall temperature, and a constant wall heat flux. Local thermal non-equilibrium is considered, and the two-equation model is used to consider heat exchange between gas and solid phases. The flow is analyzed through a non-Darcian model, in which viscous and convective terms are considered in the Darcy pressure equation. The results obtained point out the importance of the inner zone (close to the wall) analysis in the case of a constant wall heat flux. From the analysis, a new dimensionless parameter, κ, emerges, which gathers information of the transport phenomena in gas and solid phases, and it is responsible to determine the flow field.     

A hybrid GA approach for OPF with consideration of FACTS devices

This letter presents a hybrid genetic algorithm (GA) method to solve optimal power flow (OPF) in power systems incorporating flexible AC transmission systems (FACTS). In the solution process, GA is integrated with conventional OPF to select the best control parameters to minimize the total generation fuel cost and keep the power flows within the security limits. A case study using an IEEE test system is presented to demonstrate its applicability     

Determination of methanol diffusion and electroosmotic drag coefficients in proton-exchange-membranes for DMFC

Methanol diffusion and electroosmotic drag coefficients for different polymer–electrolyte-membranes have been investigated. It is essential to understand the transport phenomena of water and methanol transport in perfluoro sulfonic acid (PSA) membranes under different methanol concentrations and current densities in order to optimize cell performance and operation. The dependence of the methanol diffusion coefficient as well as the electroosmotic drag coefficient on methanol concentration and current density were observed. The results are discussed in comparison to measured values obtained by other scientific groups.     

Determination of heat transfer coefficients for cylindrical products exposed to forced-air cooling

An analytical model for estimation of transient heat transfer coefficients in forced-air precooling experiments of cylindrically shaped grapes, using a lumped capacitance approach were addressed and investigated. In order to determine transient heat transfer coefficients, the centre transient temperature measurements during forced-air precooling were used. Experiments involved cooling individual grapes in air flow without water losses. The individual grapes were instrumented with several interior thermocouples for measuring the centre transient temperature response during cooling. The transient values of the heat transfer coefficient history for five different air velocities were found to be about 21–40 W/m² K. These values were in good agreement with the values predicted using well-known Nusselt-Reynolds empirical correlation for forced convection. The present technique has the capability of determining transient heat transfer coefficients in a single transient experiment.     

Taylor dispersion coefficients for laminar,longitudinal flow past arrays of circular tubes

The problem of determining shell-side Taylor dispersion coefficients for a shell-and-tube configuration is examined in detail for both ordered as well as disordered arrangement of tubes. The latter is modeled by randomly placing N tubes within a unit cell of a periodic array. It is shown that shell-side Taylor dispersion coefficient D_T is expressed by D_T = D_M(1 + λPe²) and the coefficient λ is divergent with N, where D_M is the molecular diffusivity of solute on the shell side and Pe is the Peclet number given by aU/D_M with a and U being the radius of tube and the mean fluid velocity on the shell side, respectively. The coefficient λ depends on the spatial average and the fluid velocity weighted average of the concentration of solute on the shell side. The behavior of the coefficient λ with N arises due to logarithmically divergent nature of concentration disturbances caused by each tube in the plane normal to the axes of the tubes. An effective-medium theory is developed for determining conditionally-averaged velocity and concentration fields and hence the shell-side Taylor dispersion coefficients. Its predictions are compared with the results of rigorous numerical computations. The present study also presents formulas for determining the shell-side Taylor dispersion coefficients for square and hexagonal arrays of tubes with cell theory approximations.     

Energy policy formulation for Pakistan: An optimization approach

Pakistan is a low income, low energy consumption country. In view of the close interdependence between economic growth and energy consumption, she will need increasing energy supplies in order to maintain her economic growth. This paper develops an energy sector optimization model for the Pakistan economy, which consists of production models for five energy industries, ie oil, gas, coal, electricity and non-commercial fuels. The model is first used to forecast energy balances for the period 1975–2006. The model is then employed to formulate a long-term comprehensive energy policy for Pakistan. Finally the suggested policy is compared with the current official energy programme.     

Determination of heat transfer coefficients for hydrogen condensation in condensers of various geometries

An important task for the hydrogen isotopes separation by cryogenic distillation is to establish the shape and dimension of the column condenser and boiler in order to obtain the desired load and separation for the distillation column. In the paper we present the set-up and experimental values for the heat transfer coefficient on various types of condensers. The heat transfer coefficients were determined by measurements on liquid hydrogen flow-rate condensed on the cold surface and temperature drop between the cooling liquid and the condensate. The experiments were made for different vapor pressures and certain temperatures of the cooling liquid from the condenser. As results we determined the condensation heat transfer coefficients for different shapes and geometries of the condensers as a function of the condensate film temperature drop.     

Leak-before-break: An integrated approach for high energy piping

An integrated approach that involves system design, thermal hydraulics, materials, and fracture mechanics analyses to assure that pipe failure is highly unlikely is described. This approach is based on a leak-before-break (LBB) premise and includes through-wall flaw detectable leakage, through-wall flaw stability, and part-through-wall flaw fatigue crack propagation calculations. A successful application of LBB can reduce the amount of excessive pipe rupture restraint hardware. Assuring LBB not only reduces initial construction, future maintenance, and radiation exposure costs, but the overall safety and integrity of the plant are improved. This last benefit comes about from gaining additional insight into the piping systems and their capabilities. Details of the LBB methodology are presented here with specific examples for two pressurized water reactor lines (one inside containment fabricated of stainless steel, and the other outside containment made from ferritic steel). The application of this approach at Beaver Valley Power Station—Unit 2 indicates that pipe rupture hardware is not necessary for stainless steel lines inside containment as small as 6-in (152 mm) nominal pipe size that have passed screening criteria designed to eliminate potential problem systems (such as the feedwater system). Similarly, some ferritic steel lines as small as 3-in (76 mm) diameter (outside containment) can qualify for pipe rupture hardware elimination.