The potential for heat pumps in drying and dehumidification systems II: An experimental assessment of the heat pump characteristics of a heat pump dehumidification system using R114

An experimental heat pump dehumidifier is described. Actual coefficients of performance (COP)_A are plotted against the gross temperature lift (T_CO - T_EV) for various bypass ratios and air velocities. Interpolated values of (COP)_A for a specified temperature lift were obtained by fitting each set for various dry bulb temperatures of air leaving the humidifier using a linear equation. These values of (COP)_A are plotted against the linear velocity of the air stream approaching the evaporator at different dry bulb temperatures. The curves show a maximum of (COP)_A at approach velocities in the region of 1·6 ms^?1.     

Climatic growing conditions of Jatropha curcas L.

The massive investment in new jatropha plantations worldwide is not sufficiently based on a profound scientific knowledge of its ecology. In this article, we define the climatic conditions in its area of natural distribution by combining the locations of herbarium specimens with corresponding climatic information, and compare these conditions with those in 83 jatropha plantations worldwide.Most specimens (87%) were found in tropical savannah and monsoon climates (A_m, A_w) and in temperate climates without dry season and with hot summer (C_fa), while very few were found in semi-arid (B_S) and none in arid climates (B_W). Ninety-five percent of the specimens grew in areas with a mean annual rainfall above 944 mm year⁻¹ and an average minimum temperature of the coldest month (T_min) above 10.5 °C. The mean annual temperature range was 19.3–27.2 °C.The climatic conditions at the plantations were different from those of the natural distribution specimens for all studied climatic variables, except average maximum temperature in the warmest month. Roughly 40% of the plantations were situated in regions with a drier climate than in 95% of the area of the herbarium specimens, and 28% of the plantations were situated in areas with T_min below 10.5 °C.The observed precipitation preferences indicate that jatropha is not common in regions with arid and semi-arid climates. Plantations in arid and semi-arid areas hold the risk of low productivity or irrigation requirement. Plantations in regions with frost risk hold the risk of damage due to frost.     

Potential solar earth-water distillate yields in Africa

A model for predicting solar earth-water distillate yield for soil moisture contents up to saturation is presented. The model developed by join-point analysis for a 20 cm tall solar still with reflective interior siding is: Water Yield = C1(SR) + C2(T_MIN) + C3(T_MAX) − 30.65 + C4(T_MAX − 30.65) + C5(MC − 8.0 + C6(MC − 8.0 + I Where SR, T_MAX, T_MIN, and MC represent the total daily solar radiation, maximum and minimum daily temperatures, and soil moisture content, respectively. C1, C2, C3, C4, C5, and C6 are the regression coefficients in the predictive model, and I is the intercept. The model indicates that maximum yield can be obtained at 8% soil moisture. The equation is used to predict potential earth-water distillate yields for 8 locations in Africa. Four levels of soil moisture content (5, 10, 15, and 20% by dry weight), and 50% and 100% of clear-day solar radiation and appropriate temperature values are used. For the four tested soil moisture contents the predicted daily earth-water yields vary from a minimum of 0.56 1 m⁻²-day⁻¹ at 5% soil moisture and 50% solar radiation to a maximum of 2.12 1 m⁻²-day⁻¹ at 10% soil moisture and 100% solar radiation. Distillate yields increase as soil moisture content increases from 5 to 10%. Above 10% soil moisture, the earth-water yield decreases as the moisture content increases. Distillate yield varies with soil moisture in the following manner: Y_10% > Y_15% > Y_20% > Y_5%, where Y is the predicted yield.     

Validation of five global radiation models with measured daily data in China

Two sunshine based and three air temperature based global radiation models are calibrated using daily data in Jan. 1 1994–Dec. 31 1998 at 48 stations all over China. The Nash–Sutcliffe equation (NSE) is used as the model evaluation criterion. The sunshine based models are suitable for daily global radiation estimation. The averaged NSE value of the Angström model is 0.83, and the maximum value is 0.91. The maximum NSE value of the Bahel model is 0.92 with an averaged value of 0.84. The models that use air temperature as the input variable are not suitable for daily global radiation estimation in China. The averaged NSE values of the three air temperature based models (Bristow–Campbell model, Allen model and Hargreaves model) are not larger than 0.47. A logarithmic relationship between the daily global radiation/daily extra-terrestrial solar radiation (R_G/R_A) and the temperature difference between the maximum and minimum daily air temperature (T_M−T_m) is found in the present study. A new daily global radiation model that is a function of R_A, sunshine hours and T_M−T_m is designed, which gives an averaged NSE value of 0.85 and a maximum value of 0.92.     

Experimental and numerical determination of a new wall-heat-gain function

A new ‘wall-heat-gain function’ is developed, which provides the heat entering a space through a wall under periodic outdoor conditions and constant indoor air temperature. The proposed function, which is much simpler than the well-known conduction transfer function, contains three coefficients U, w₁ and w₂ characterizing the wall and three parameters T_M, T_C, and T_s characterizing the outdoor conditions (temperature and solar radiation). The wall coefficients U, w₁, and w₂ may be determined numerically by the use of the finite difference method or experimentally in the case of existing (built) walls of unknown properties. A related experimental set up has been developed. Ready to use values of the wall coefficients are provided for 15 typical wall constructions. The climatic parameters T_M, T_C and T_s either are available for some locations or may be easily calculated by least-squares fitting to local climatological data. In a modified form of the proposed function the three climatic parameters are reduced to two, which are available from ASHRAE for about 1000 stations around the world. The accuracy of the proposed wall heat gain function is very good for practical applications as proved by comparisons with corresponding finite difference solutions.     

Estimation of monthly solar radiation from measured temperatures using support vector machines – A case study

Solar radiation is the principal and fundamental energy for many physical, chemical and biological processes. However, it is measured at a very limited number of meteorological stations in the world. This paper presented the methods of monthly mean daily solar radiation estimation using support vector machines (SVMs), which is a relatively new machine learning algorithm based on the statistical learning theory. The main objective of this paper was to examine the feasibility of SVMs in estimating monthly solar radiation using air temperatures. Measured long-term monthly air temperatures including maximum and minimum temperatures (T_max and T_min, respectively) were gathered and analyzed at Chongqing meteorological station, China. Seven combinations of air temperatures, namely, (1) T_max, (2) T_min, (3) T_max ? T_min, (4) T_max and T_min, (5) T_max and T_max ? T_min, (6) T_min and T_max ? T_min, and (7) T_max, T_min, and T_max ? T_min, were served as input features for SVM models. Three equations including linear, polynomial, and radial basis function were used as kernel functions. The performances were evaluated using root mean square error (RMSE), relative root mean square error (RRMSE), Nash-Sutcliffe (NSE), and determination coefficient (R²). The developed SVM models were also compared with several empirical temperature-based models. Comparison analyses showed that the newly developed SVM model using T_max and T_min with polynomial kernel function performed better than other SVM models and empirical methods with highest NSE of 0.999, R² of 0.969, lowest RMSE of 0.833 MJ m^?2 and RRMSE of 9.00%. The results showed that the SVM methodology may be a promising alternative to the traditional approaches for predicting solar radiation where the records of air temperatures are available.     

Calendar- and cycle-life studies of advanced technology development program generation 1 lithium-ion batteries

This paper presents the test results and life modeling of special calendar- and cycle-life tests conducted on 18650-size generation 1 (Gen 1) lithium-ion battery cells (nominal capacity of 0.9 Ah; 3.0–4.1 V rating) developed to establish a baseline chemistry and performance for the Department of Energy sponsored advanced technology development (ATD) program. Electrical performance testing was conducted at the Argonne National Laboratory (ANL), Sandia National Laboratory (SNL) and the Idaho National Engineering and Environmental Laboratory (INEEL).As part of the electrical performance testing, a new calendar-life test protocol was used. The test consisted of a once per day discharge and charge pulse designed to have minimal impact on the cell yet establish its performance over a period of time such that the calendar-life of the cell could be determined. The calendar-life test matrix included two states-of-charge (SOCs) (i.e. 60 and 80%) and four test temperatures (40, 50, 60 and 70 °C). Discharge and regen resistances were calculated from the test data. Results indicate that both the discharge and regen resistances increased non-linearly as a function of the test time. The magnitude of the resistances depended on the temperature and SOC at which the test was conducted. Both resistances had a non-linear increase with respect to time at test temperature. The discharge resistances are greater than the regen resistances at all of the test temperatures of 40, 50, 60 and 70 °C. For both the discharge and regen resistances, generally the higher the test temperature, the lower the resistance.The measured resistances were then used to develop an empirical model that was used to predict the calendar-life of the cells. This model accounted for the time, temperature and SOC of the batteries during the calendar-life test. The functional form of the model is given by: R(t,T,SOC)=A(T, SOC)F(t)+B(T, SOC), where t is the time at test temperature, T the test temperature and SOC the SOC of the cell at the start of the test. A(T, SOC) and B(T, SOC) are assumed to be functions of the temperature and SOC; F is assumed to only be a function of the time at test temperature. Using curve-fitting techniques for a number of time-dependent functions, it was found that both the discharge and regen resistances were best correlated with F(t) having a square-root of test time dependence. These results led to the relationship for the discharge and regen resistances: R(t,T,SOC)=A(T, SOC)t^1/2+B(T, SOC). The square-root of time dependence can be accounted for by either a one-dimensional diffusion type of mechanism, presumably of the lithium-ions or by a parabolic growth mechanism for the growth of a thin-film solid electrolyte interface (SEI) layer on the anode and/or cathode. The temperature dependence of the resistance was then investigated using various model fits to the functions A(T, SOC) and B(T, SOC). The results of this exercise lead to a functional form for the temperature dependence of the fitting functions having an Arrhenius-like form: A(T,SOC)=a(SOC){exp[b(SOC)/T]} and B(T,SOC)=c(SOC){exp[d(SOC)/T]}, where a and c are constants, and b and d are related to activation energy (E_b and E_d) by using the gas constant (R) such that b=E_b/R and d=E_d/R. The functional form, therefore, for the discharge and regen resistances, including the SOC, is then: R(t,T,SOC)=a(SOC){exp[b(SOC)/T]}t^1/2+c(SOC){exp[d(SOC)/T]}. The a, b, c and d parameters are explicitly shown as being functions of the SOC. However, due to the lack of testing at SOC values other than 60 and 80% SOC, the exact form of the SOC dependence could not be determined from the experimental data. The values of a, b, c and d were determined, thus permitting the function R(t, T, SOC) to be used to correlate the discharge and regen data and to predict what the resistances would be at different test times and temperatures.This paper also presents, discusses and models the results of a special cycle-life test conducted for a period of time at specified temperatures of 40, 50, 60 and 70 °C. This test, consisting of specified discharge and charge protocols, was designed to establish the cycle-life performance of the cells over a time interval such that their cycle-life could be determined. The cycle-life test was conducted at 60% SOC, with SOC swings of Δ3, Δ6 and Δ9%. During the cycle-life test, the discharge and regen resistances were determined after every 100 test cycles. The results of the cycle-life testing indicate that both the discharge and regen resistances increased non-linearly as a function of the test time at each Δ% SOC test. The magnitude of the resistances and the rate at which they changed depended on the temperature and Δ% SOC at which the test was conducted. Both resistances had a non-linear increase with respect to time at test temperature, i.e. as the number of test cycles increased the discharge and regen resistances increased also. For a given Δ% SOC test, the discharge resistances are greater than the regen resistances at all of the test temperatures of 40, 50, 60 and 70 °C. For both the discharge and regen resistances, generally the higher the test temperature, the lower the resistance. At each of the four test temperatures, the magnitude of the discharge and regen resistances was generally in the following order: Δ3% SOC>Δ9% SOC>Δ6% SOC, but the ordering was dependent on test time.A model was also developed to account for the time, temperature, SOC and Δ% SOC of the batteries during the cycle-life test. The functional form of the model is given by R(t,T,SOC,Δ% SOC)=A(T, SOC, Δ% SOC)F(t)+B(T, SOC, Δ% SOC) where t is the time at test temperature, T the test temperature, SOC the SOC of the cell at the start and end of the test and Δ% SOC the SOC swing during the test. A(T, SOC, Δ% SOC) and B(T, SOC, Δ% SOC) are assumed to be functions of the test temperature, SOC and Δ% SOC swing. F(t) is assumed to only be a function of the test time at test temperature. Using curve-fitting techniques for a number of time-dependent functions, it was found that both the discharge and regen resistances were best correlated by a square-root of test time dependence. These results led to the relationship for the discharge and regen resistances having the form R(t,T,SOC,Δ% SOC)=A(T, SOC, Δ% SOC)t^1/2+B(T, SOC, Δ% SOC). This model is essentially the same as used to analyze the calendar-life test data. The temperature dependence of the resistance was then investigated using various model fits to the functions A(T) and B(T). The results of this exercise lead to a functional form for the functions having again an Arrhenius-like form: A(T)=a[exp(b/T)] and B(T)=c[exp(d/T)] where a and c are constants, and b and d are related to activation energies. The functional form, therefore, for the discharge and regen resistances including the SOC and Δ% SOC is R(t,T,SOC,Δ% SOC)=a(SOC, Δ% SOC){exp[b(SOC, Δ% SOC)/T]}t^1/2+c(SOC, Δ% SOC){exp[d(SOC, Δ% SOC)/T]}. The a, b, c and d parameters are explicitly shown as being functions of the SOC and the Δ% SOC. However, due to the lack of testing at SOC values other than 60% SOC, the exact form of the SOC dependence could not be determined from the experimental data. In addition, no model was found that consistently correlated the observed resistance changes with the Δ% SOC of the tests. Eliminating the SOC and Δ% SOC from the resistance function, the function R(t, T) was then used to correlate the discharge and regen resistances data. This model also allows the prediction of what the resistances would be at different test times at a particular Δ% SOC test condition and temperature.     

Gaseous diffusion coefficients of methyl bromide and methyl iodide into air,nitrogen, and oxygen

The gaseous diffusion coefficients of methyl bromide (CH₃Br) and methyl iodide (CH₃I) into dry air, nitrogen, and oxygen have been measured in the temperature range 303–453 K and at atmospheric pressure via the Taylor dispersion method. Both for methyl bromide and methyl iodide, the diffusion coefficients do not vary in practice on substituting pure nitrogen or oxygen for dry air. The diffusion coefficients for methyl iodide are systematically smaller than those for methyl bromide by about 11%. For the methyl iodide‐oxygen system, the effect of the thermal decomposition of methyl iodide has been observed at 453 K. The present results can be reproduced well by the functional form D = AT^B, where D (cm²s^?1) is the diffusion coefficient at 101 325 Pa (1 atm) and T (K) is the absolute temperature. The constants A and B are as follows: methyl bromide‐(air, nitrogen, oxygen), A = 5.57 × 10^?6, B = 1.76; methyl iodide‐(air, nitrogen, oxygen), A = 5.26 × 10^?6, B = 1.75. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20255     

Global solar radiation estimation from measurements of visibility and air temperature extremes

Solar radiation is the main source of energy for the survival of life and its associated activities. It is important to know accurate solar radiation value in areas such as agricultural activities, solar energy systems, heating, and meteorology. In this study, we present a model for the estimation of solar radiation value with other meteorological parameters in cases where solar radiation cannot be measured or not available. This model is based on the relationship between solar radiation and measured air temperature and visibility extremes. As is known, the incident global solar radiation is attenuated by clouds, aerosols, ozone layer, water vapor, etc.. In the model, the attenuation of the solar radiation is expressed by dew point temperature, visibility, and the maximum and minimum air temperatures. Dew-point temperature refers to the effect of water vapor on solar radiation, air temperature extremes are used to signify cloudiness. Visibility also gives the effect on the attenuation of solar radiation by air pollutants and aerosols in the model. The model was applied to the data taken from meteorological stations in Turkey. Error analysis was performed and compared with the models in the literature and satisfactory results were obtained.

Abbreviations H: Daily total global solar radiation, units of MJ ? m^?2 ? day^?1; H₀: Extraterrestrial solar radiation, units of MJ ? m^?2 ? day^?1; H_m: Measured daily total global solar radiation, units of MJ ? m^?2 ? day^?1; H_c: Calculated daily total global solar radiation, units of MJ ? m^?2 ? day^?1; T_min: Daily minimum temperature, units of °C; T_max: Daily maximum temperature, units of °C; RH: T_dew: Relative humidity, units of %rh; Dew-point temperature, units of °C     

Parametric study on the thermoelectric conversion performance of a concentrated solar‐driven thermionic‐thermoelectric hybrid generator

A concentrated solar‐driven thermionic‐thermoelectric hybrid generator composed of solar heat collector, thermionic generator (TIG), thermoelectric generator (TEG), and radiator is introduced in this paper. A theoretical model of thermoelectric conversion performance for the hybrid generator is built up based on the heat source of the concentrated solar radiation rather than isothermal heat source. Based on the model, the impacts of related parameters on the internal temperature distributions, output power, and efficiency have been discussed. Moreover, the optimal operating conditions of the TIG‐TEG hybrid device at its maximum output power and efficiency have been determined. Results show that when cascading the TEG with the TIG, there is very little change of the TIG cathode temperature in most conditions, namely, T_C ≈ T_C′. Meanwhile, the anode temperature becomes higher, and the TEG cold end temperature T₂ is close to the anode temperature T_A′ for the single TIG system, ie, T_A > T_A′ ≈ T₂. In theory, the optimal concentrated solar radiation I₀ for the maximum output power P_max and the maximum efficiency η_max differs, which are I_0,P = 2.5 × 10⁶ W/m² and I_0,η = 2 × 10⁶ W/m², respectively, whereas the output power and efficiency of the TIG‐TEG hybrid system simultaneously reach their maximum values when the optimal TIG anode temperature T_A,opt = 1025 K, the optimal TIG output voltage V_opt = 2 V, and the optimal ratio of load resistance to internal resistance (R₂/R)_opt = 2. However, in practice, the parameter values of I₀, Φ_A, and T_A should be strictly controlled under 1.8 × 10⁶ W/m², 1.4 eV, and 660 K, respectively. Generally, the maximum output power and efficiency of the hybrid TIG‐TEG system are, respectively, 35% and 4% higher than that of the single TIG.     

Exergetic efficiency of high‐temperature‐lift chemical heat pump (CHP) based on CaO/CO2 and CaO/H2O working pairs

The use of reversible chemical reactions in recuperation of heat has gained significant interest due to higher magnitude of reaction heat compared to that of the latent or sensible heat. To implement chemical reactions for upgrading heat, a chemical heat pump (CHP) may be used. A CHP uses a reversible chemical reaction where the forward and the reverse reactions take place at two different temperatures, thus allowing heat to be upgraded or degraded depending on the mode of operation. In this work, an exergetic efficiency model for a CHP operating in the temperature‐level amplification mode has been developed. The first law and the exergetic efficiencies are compared for two working pairs, namely, CaO/CO₂ and CaO/H₂O for high‐temperature high‐lift CHPs. The exergetic efficiency increases for both working pairs with increase in task, T_H, decrease in heat source, T_M, and increase in condenser, T_L, temperatures. It is also observed that the difference in reaction enthalpies and specific heats of the involving reactants affects the extent of increase or decrease in the exergetic efficiency of the CHP operating for temperature‐level amplification. Copyright © 2012 John Wiley & Sons, Ltd.     

Experimental investigation of transient thermal characteristics of a dry friction clutch using alternative friction materials under different operating conditions

The thermal characteristics of four types of dry friction clutch materials (LUK, G95, HCC, and Tiger) are investigated experimentally and numerically in the present work under different working conditions; such as initial sliding angular velocity (ω_ro), torque (T), and sliding time (t_s). The temperature distributions over a cross-section of friction clutch elements (pressure plate and flywheel) are investigated and optimized during the sliding period (heating phase), and full engagement period (cooling phase). The effect of alternative frictional materials lining of a clutch disc on the thermal behavior of the sliding system under different operating conditions (different angular velocities, torques, and sliding periods) is investigated experimentally. The results showed that the maximum effect on the temperature values occurred when applying maximum torque (4.5 kg·m), maximum initial rotational speed (1200 rpm), slipping period (30 s). However, the temperature values at interface contact decrease when decreasing all the above input conditions values to (2.5 kg·m, 690 rpm, and slipping period to 8 s). The results showed that the temperature reduced (53%) from (180.4°C) for applied torque 4.5 kg·m with initial rotational speed (1200 rpm) and slip period (30 s) to (83.3°C) when applied torque 2.5 kg·m, initial rotational speed (680 rpm) and slip period (8 s) for clutch disc (LUK). It was obtained the same behavior for the other three discs (G95, HCC, and Tiger), but with different values of temperatures. The results show that the temperatures of the pressure plate interface (T_max = 159.1°C) are higher than those at the flywheel interface (T_max = 152.7°C), due to the low thermal capacity of pressure plate compared to the flywheel when using G95 frictional material. The experimental optimization results showed that the highest temperatures were obtained when using friction clutch disc (LUK), and minimum temperature when using (HCC) disc, around (20%) reduction when replaced (LUK) material with (HCC) under the same working conditions (T = 4.5 kg·m, ω_ro = 1200 rpm, and t_s = 30 s).     

Computations of the optical properties of metal/insulator-multilayers for solar thermal applications

A computer program has been developed to calculate the optical properties of metal/ insulator-multilayer systems from complex refractive indices of corresponding bulk materials. In addition, interdiffusion in multilayer structures could be detected by fitting experimental reflectance spectra. Among M/Al₂O₃-systems, M = (Ta, Mo, W, orPt), optimum multilayer structures were evaluated as selective absorbers for solar thermal applications at various operating temperatures. The influence of the kind of metal, the modulation number, and the layer thicknesses on the solar thermal performance were studied. At temperatures of T_B ≈ 1100 K and 100-fold sunlight concentration Mo/Al₂0₃- and W/Al₂O₃-multilayers with a modulation number of 6.5 and metal layer thicknesses of a few nm exhibited an absorptance of merit up to A_m ≈ 0.8. Lower temperatures favoured Pt-systems (A_m ≈ 0.94 at 400 K) and higher temperatures Ta-systems (A_m ≈ 0.72 at 1400 K).     

Convective‐radiative fins with simultaneous variation of thermal conductivity,heat transfer coefficient,and surface emissivity with temperature

This paper is a numerical study of thermal performance of a convective‐radiative fin with simultaneous variation of thermal conductivity, heat transfer coefficient, and surface emissivity with temperature. The convective heat transfer is assumed to be a power function of the local temperature between the fin and the ambient which allows simulation of different convection mechanisms such as natural convection (laminar and turbulent), boiling, etc. The thermal conductivity and the surface emissivity are treated as linear functions of the local temperature between the fin and the ambient which provide a satisfactory representation of the thermal property variations of most fin materials. The thermal performance is governed by seven parameters, namely, convection–conduction parameter Nc, radiation–conduction parameter Nr, thermal conductivity parameter A, emissivity parameter B, the exponent n associated with convective heat transfer coefficient, and the two temperature ratios, θ_a and θ_s, that characterize the temperatures of convection and radiation sinks. The effect of these parameters on the temperature distribution and fin heat transfer rate are illustrated and the results interpreted in physical terms. Compared with the constant properties model, the fin heat transfer rate can be underestimated or overestimated considerably depending on the values of the governing parameters. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20408     

Calculation of the second virial coefficient of nonspherical molecules: Revisited

An accurate Hartree–Fock dispersion individually damped (HFD-ID) potential type improved by Boyes for argon has been used as a core potential to calculate both the spherical and nonspherical contributions to the second virial coefficient of simple molecules. The auxiliary functions that occur in the perturbation terms for calculating the nonspherical contributions, have been calculated numerically and are tabulated over a wide range of temperatures from T^*=0.5 to T^*=10, where T^*=kT/ and is the potential well-depth. By fitting the well-depth and the position of the minimum in the core potential, we have calculated the second virial coefficient of N₂, O₂, CO, NO, and CO₂ over the whole temperature range reported in the literature. The calculated results are compared with the large body of experimental data in the literature, and with the pervious calculations by Boushehri et al. (1987). The agreement with both experimental data and theoretical calculations is quite good.     

Sensitivity analysis of transfer functions of laminar flames

The sensitivity of laminar premixed methane/air flames responses to acoustic forcing is investigated using direct numerical simulation to determine which parameters control their flame transfer function. Five parameters are varied: (1) the flame speed s_L, (2) the expansion angle of the burnt gases α, (3) the inlet air temperature T_a, (4) the inlet duct temperature T_d and (5) the combustor wall temperature T_w. The delay of the flame transfer function is computed for the axisymetric flames of Boudy et al. [1] and the slot flames of Kornilov et al. [2]. Stationary flames are first computed and compared to experimental data in terms of flame shape and velocity fields. The flames are then forced at different frequencies. Direct numerical simulations reproduce the flame transfer functions correctly. The sensitivity analysis of the flame transfer function is done by changing parameters one by one and measuring their effect on the delay. This analysis reveals that the flame speed s_L and the inlet duct temperature T_d are the two parameters controlling the flame delay and that any precise computation of the flame transfer function delay must first have proper models for these two quantities.     

Influence of thermal ageing on the stability of polymer bulk heterojunction solar cells

A new approach is presented in order to improve the thermal stability of polymer: [6-6]-phenyl C₆₁ butyric acid methyl ester (PCBM) bulk heterojunction solar cells. The central idea in this approach is the use of a polymer with high glass transition temperature (T_g), well above the normal operating temperatures of the devices. In this paper, a PPV-derivative with a T_g of 150 °C was used as an electron donor and the thermal stability of the obtained solar cells was compared with solar cells based on the reference material poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) with a T_g of 45 °C. The use of the material with higher glass transition temperature resulted in a significant improvement of the thermal stability of the photovoltaic parameters. Furthermore, a systematic transmission electron microscope (TEM) study demonstrates that the better thermal stability of performance coincides with a more stable active layer morphology. Both improvements are attributed to the reduced free movement of the electron donor material (PCBM) within the active layer of the solar cell.     

Dissipative model of film boiling crisis

A dissipative model of the film boiling crisis based on the variational hypothesis of nonequilibrium phase change is presented. Transfer systems—characteristic for film and transition boiling of a liquid droplet on a plane horizontal and isothermal heating surface—were constructed. The value of the minimum film boiling temperature T_p,min was calculated from the criterion of equality of local potentials for two competitive transfer systems. The curves p = p(T_p,min) for hydrodynamic and thermodynamic models of the film boiling crisis for water have been determined and compared with the results achieved for the dissipative model.     

Cooling performance and efficiency of night sky radiators

A theoretical equation was derived to predict the surface temperature of night sky radiators as a function of power, Q, from radiator properties and sky conditions. The power of an ideal radiator, R_I, which is perfectly black in the 8–14 μm atmospheric window, perfectly reflective outside the window and has a transparent cover was used to define radiator efficiency as . Plots of against , where T_a and T_s are the air and radiator surface temperatures, were primarily dependent on radiator properties and only slightly on environmental conditions. These curves provide a means to compare different radiators and to aid in the design and prediction of performance of night sky radiators.Performance measurements were obtained with three night sky radiators constructed with surfaces of aluminum, white TiO₂ paint, and black paint covered with polyethylene. Similar measurements were also obtained with a fourth radiator that had an uncovered black paint surface. Depressions below air temperature for Q = 0 of 6 and 2.5°C were observed with the aluminum and the black-uncovered radiators at vapor pressures of 0.5 and 2 kPa, respectively. Depressions of the white and black paint covered radiators were about 11 and 6°C at vapor pressures of 0.5 and 2 kPa. Fair agreement with theory was achieved. Calculations of cooling losses from various radiators for the Phoenix, AZ, climate were made. Generally the losses were too small for practical use in July and August, but had potential for other months. The procedure presented can be used to predict the feasibility of radiator use for other application temperatures, climates and radiator properties.     

Energy delivery of solar thermal collectors in Zimbabwe

The long-term annual thermal energy delivery per unit of collector area of commonly used collector types and configurations, for a range of operating temperatures, are calculated for representative locations in Zimbabwe. A well-known model found in the literature is the basic tool of analysis, the only modifications being, the use of a locally-derived correlation of the monthly average diffuse fraction of hemispherical radiation to the monthly average clearness index, and the use of temperature-dependent collector heat loss coefficients. The results are presented as plots of annual specific thermal energy output against collector receiver temperature gain (T_r−T_a).The results, though founded on a number of simplifying assumptions on some collector parameters, provide a sound basis for the economic evaluation of solar thermal applications in Zimbabwe.