The features of ignition and combustion of composite propane-hydrogen fuel: Modeling study

The extensive numerical analysis of the features of self ignition and formation of NO and CO during combustion of blended fuel, consisting of propane and hydrogen, with air is considered on the basis of extended detailed kinetic model involving both high and low temperature submechanisms of propane oxidation. It has been shown that for the blended C₃H₈–H₂ fuel there exists the temperature region, where the ignition of the C₃H₈–H₂–air mixture occurs faster compared to pure propane. However, this region is not broad enough and has low and high temperature boundaries (T_b and T_h, respectively). At the initial temperature of fuel–air mixture T₀ < T_b, the induction time of blended C₃H₈–H₂ fuel is greater than that of pure propane and, at T₀ > T_h, the admixture of a small amount of propane (1 ∼ 5% per volume) to hydrogen accelerates the ignition. The values of T_b and T_h depend on the composition of blended fuel and initial pressure. It has been revealed that the addition of hydrogen to propane increases the flame speed and extends the flammability thresholds both in fuel-lean and in fuel-rich regions, but doesn't result in the substantial change of the concentrations of main pollutants NO and CO in the combustion exhaust. However, the addition of hydrogen to fuel-lean propane–air mixture allows one to provide the stable combustion of leaner fuel–air mixture and, thus, to reduce notably the emission of NO and CO compared to that for the combustion of pure propane–air mixture.     

Finite element simulation of hydromagnetic convective flow in an obstructed cavity

The hydromagnetic natural convective flow and heat transfer characteristics in a square cavity with a solid circular heated obstacle located at the center have been investigated numerically. The left vertical surface of the cavity is uniformly heated of temperature T_c and other three surfaces are adiabatic. The obstacle consists of constant heat T_h. Under all circumstances the condition T_h > T_c is maintained. The physical problem is represented mathematically by sets of governing equations and the developed mathematical model is solved by employing Galerkin weighted residual finite element simulation. The behavior of the fluid in the ranges of Prandtl number (0.073-2.73), Hartmann number (0-50) and Joule heating parameter (1-7) is explained in details. It is found that the flow and temperature fields are strongly dependent on the above stated parameters for the ranges considered. The variation of the average Nusselt number (Nu) for various Prandtl number (Pr) is also presented.     

Investigation of heat transfer in square porous-annulus

The current study is focused to analyze the heat transfer characteristics in a porous duct. The mathematical model of heat transfer in a porous duct was solved by converting the governing partial differential equations into a set of algebraic equations with the help of finite element method. A simple three noded triangular element is used to mesh the duct domain. The current problem consists of a square duct with outer walls being exposed to hot temperature T_h, and inner walls subjected to cool temperature T_c. Emphasis is given to investigate the effect of width ratio of cavity on heat and fluid flow characteristics inside the porous medium. The results are reported for various duct width ratios, Rayleigh number etc. It is found that the Nusselt number increases with increase in height of cavity along the vertical walls of duct; however the Nusselt number for certain values of duct ratio oscillates along the width of the porous medium at bottom wall of the cavity.     

Numerical study of compressible convective heat transfer with variations in all fluid properties

The effects of property variations in single-phase laminar forced micro-convection with constant wall heat flux boundary condition are investigated in this work. The fully-developed flow through micro-sized circular (axisymmetric) geometry is numerically studied using two-dimensional continuum-based conservation equations. The non-dimensional governing equations show significance of momentum transport in radial direction due to μ(T) variation and energy transport by fluid conduction due to k(T) variation. For the case of heated air, variation in C_p(T) and k(T) causes increase in Nu. This is owing to: (i) reduction in T_w, (T_w ? T_m), and (?T/?r)_w and (ii) change in ?T_m/?z results in axial conduction along the flow. The effects of ρ(p,T) and μ(T) variation on convective-flow are indirect and lead to: (i) induce radial velocity which alters u(r) profile significantly and (ii) change in (?u/?r)_w along the flow. It is proposed that the deviation in convection with C_p(T), k(T) variation is significant through temperature field than ρ(p,T), μ(T) variation on velocity field. It is noted that Nu due to variation in properties differ from invariant properties (Nu = 48/11) for low subsonic flow.     

Long-term behavior of cooling fluid in a vertical cylinder

The long-term behavior of cooling an initially quiescent isothermal Newtonian fluid in a vertical cylinder by unsteady natural convection with a fixed wall temperature has been investigated in this study by scaling analysis and direct numerical simulation. Two specific cases are considered. Case 1 assumes that the fluid cooling is due to the imposed fixed temperature on the vertical sidewall whereas the top and bottom boundaries are adiabatic. Case 2 assumes that the cooling is due to that on both the vertical sidewall and the bottom boundary whereas the top boundary is adiabatic. The long-term behavior of the fluid cooling in the cylinder is well represented by T_a(t), the average fluid temperature in the cylinder at time t, and the average Nusselt number on the cooling boundary. The scaling analysis shows that for both cases θ_a(τ) scales as , where θ_a(τ) is the dimensionless form of T_a(t), τ the dimensionless time, A the aspect ratio of the vertical cylinder, Ra the Rayleigh number, and C a proportionality constant. A series of direct numerical simulations with the selected values of A, Ra, and Pr (Pr is the Prandtl number) in the ranges of 1/3 ? A ? 3, 6 × 10⁶ ? Ra ? 6 × 10¹⁰, and 1 ? Pr ? 1000 have been carried out for both cases to validate the developed scaling relations, and it is found that these numerical results agree well with the scaling relations. These numerical results have also been used to quantify the scaling relations and it is found that C = 1.287 and 1.357 respectively for Case 1 and 2 with Ra, A and Pr in the ranges of 1/3 ? A ? 3, 6 × 10⁶ ? Ra ? 6 × 10¹⁰, and 1 ? Pr ? 1000.     

Effects of temperature on the location of the gas–liquid interface in a PEM fuel cell

The objective of this study is to investigate the location of the gas–liquid interface at various temperatures in a polymer electrolyte membrane fuel cell under non-isothermal conditions. A mathematical model, coupled with the electrochemical process, two-phase flows, species transfer, and heat transfer is employed. A finite volume-based CFD approach is applied to investigate the species transport behavior in a fuel cell. The effects of two model parameters, namely cell temperature (T_cell) and humidification temperature (T_h), on the gas–liquid interface and cell performance are presented. Simulation results indicate that variations of these two parameters influence the location of the gas–liquid interface, the cell performance, and the distribution of liquid water saturation. At lower cell temperatures, the gas–liquid interface moves toward the inlet port of the channel when the humidification temperature is greater than the cell temperature. Therefore, the cell performance decreases as the liquid water clogs the passage for the transport of oxygen. Furthermore, these two factors are closely related to the membrane temperature distribution. Obvious variations in magnitude are seen at a cell temperature of 323 K and a humidification temperature of 343 K.     

The reactive thermal conductivity for a two-temperature plasma

Reliable plasma thermodynamic and transport properties are required for the numerical simulation of thermal plasma systems. Although many databases for the thermal plasma properties at the local thermodynamic equilibrium (LTE) state have been compiled, the database for the two-temperature (2-T) plasma is still far from completeness. There exits considerable confusion in the literature concerning how to calculate the thermodynamic and transport properties, including the reactive thermal conductivity, for the 2-T plasma. In this paper, a detailed derivation for the reactive thermal conductivity of the 2-T argon plasma is presented using two different approaches. The present calculated results for the reactive thermal conductivity are identical to those due to Hsu [5] for the special case of LTE plasma, but are different when the electron temperature is higher than the heavy-particle temperature, the difference increases with increasing electron/heavy-particle temperature ratio, θ(=T_e/T_h), and becomes quite significant at high θ.     

Heat transfer characteristics of a partially insulated tube with an impulsive temperature rise

The heat transfer for a laminar forced convection inside a two dimensional planar symmetric duct is analyzed. The fluid passage is formed by two parallel plates, and flow is fully developed and incompressible. Flow is isothermal to a position x_o = 0, where the wall temperature jumps impulsively to T₁ > T₀ and remains at this value up to the position x₁, where it jumps back T₀. The problem is considerably simplified by introducing a transformation to reduce the heat transfer problem to the standard thermal entrance region problem for flow between parallel plates. Various heat transfer characteristics for different values of Prandtl and Nusselt numbers are analyzed and found to be physically realistic.     

The calculation of turbulent boundary layers with foreign gas injection

Numerical solutions have been obtained for flat plate compressible turbulent boundary layers of air with foreign gas injection, on the hypothesis that the momentum equation is coupled to the species and energy equations only through spatial variations of the mean density and viscosity. The finite difference calculation procedure incorporates the “ω²” transformation with central differencing. The injected species include H₂, He, CH₄, H₂O, CO, Air, CO₂, Freon 12, Xe and CCl₄; temperature ratios T_s/T_e range from 0.2 to 2.0 while the Mach numbers range from 0 to 6. Thermodynamic properties are estimated allowing for variable specific heats; for the transport properties, kinetic theory with the Lennard-Jones potential is used. Thermal diffusion and diffusional conduction are ignored. Eddy viscosity models for the inner region were evaluated by comparison with experimental data for blown constant property flows, and a “best” model selected for the parametric calculations. The turbulent Schmidt and Prandtl numbers were taken as constants, respectively 0.8 and 0.9. The results show the important role played by density variations across the boundary layer; the mass and heat transfer Stanton numbers for the heaviest injectants actually increase with injection, and the light injectants are found to be more effective in reducing skin friction and heat transfer than experimental data indicate. Agreement with experiment for low speed flows is shown to be generally satisfactory; also, the predicted influence of Mach number on skin friction is found to be consistent with experiment.     

Heat generation effects in natural convection inside a porous annulus

The interplay between internal heat generation and externally driven natural convection inside a porous medium annulus is studied in detail using numerical methods. The axisymmetric domain is bounded with adiabatic top and bottom walls and differentially heated side walls sustaining steady natural convection of a fluid with Prandtl number, Pr = 5, through a porous matrix of volumetric porosity, ? = 0.4. The generalized momentum equation with Brinkman–Darcy–Forchheimer terms and the local thermal non-equilibrium based two-energy equation model are solved to determine the flow and the temperature distribution. Beyond a critical heat generation value defined using an internal Rayleigh number, Ra_I,cr^?, the convection transits from unicellular to bicellular mode, as the annulus T_max becomes higher than the fixed hot-wall temperature. The Ra_I,cr^? increases proportionately when the permeability based external Rayleigh number Ra_E^? and the solid–fluid thermal conductivity ratio γ are independently increased. A correlation is proposed to predict the overall annulus Nu as a function of Ra_E^?, Ra_I^?, Da and γ. It predicts the results within ± 20% accuracy.     

Validation of three dimensional film cooling modeling on convex surface for gas turbine blade

In this study, three dimensional computational predictions on the film cooling performance of single row and simple cylinder on the convex surface have been studied and compared with corresponding experimental data reported in the literature to validate the model. This computational prediction serves as the baseline for future studies of optimization in determining the film cooling effectiveness. Realizable κ–? turbulent model has been employed and energy equation has been solved. Grid independence study has been fulfilled using two kinds of meshing approach for the plenum and the cooling holes. Results of grid independence study showed that fine meshed plenum and cylinders of tetrahedral grids case have provide a good agreement with the related experimental data. Study of temperature ratio between the coolant and mainstream hot gas T_c/T_g has been performed using four values of temperature ratios that are 0.5, 0.6, 0.7, and 0.8. In all of these tests the mainstream duct of the models was generated with multigrid hexahedral mesh. Based on the heat-mass transfer analogy, results of this study showed good agreement of the film cooling effectiveness and temperature distribution in comparison to the related experimental data. The case in which combination of both plenum and cylinders in one volume with tetrahedral fine mesh generation and temperature ratio of T_c/T_g = 0.6 was found to be in good agreement with the experimental data among all of the other models. Computational prediction results have found an agreement with the experimental data, thus the approach is verified.     

Solutions of Fourier’s equation appropriate for experiments using thermochromic liquid crystal

In transient heat-transfer experiments, the time to activate the thermochromic liquid crystal (TLC) can be used to evaluate h, the heat transfer coefficient. Most experimenters use the solution of Fourier’s equation for a semi-infinite substrate with a step-change in the temperature of the fluid to determine h. The ‘semi-infinite solution’ can also be used to determine T_ad, the adiabatic surface temperature, but this is an error-prone method suitable only for experiments with relatively large values of Bi, the Biot number. For Bi > 2, which covers most practical cases, more accurate results could be achieved using a composite substrate of two materials. Using TLC to determine the temperature–time history of the surface of the composite substrate, h and T_ad could be computed from the numerical solution of Fourier’s equation. Alternatively, h and T_ad could be determined analytically from a combination of the semi-infinite and steady-state solutions.     

Assessment of thermoelectric module with nanofluid heat exchanger

For applications such as cooling of electronic devices, it is a common practice to sandwich the thermoelectric module between an integrated chip and a heat exchanger, with the cold-side of the module attached to the chip. This configuration results thermal contact resistances in series between the chip, module, and heat exchanger. In this paper, an appraisal of thermal augmentation of thermoelectric module using nanofluid-based heat exchanger is presented. The system under consideration uses commercially available thermoelectric module, 27 nm Al₂O₃–H₂O nanofluid, and a heat source to replicate the chip. The volume fraction of nanofluid is varied between 0% and 2%. At optimum input current conditions, experimental simulations were performed to measure the transient and steady-state thermal response of the module to imposed isoflux conditions. Data collected from the nanofluid-based exchanger is compared with that of deionized water.Results show that there exist a lag-time in thermal response between the module and the heat exchanger. This is attributed to thermal contact resistance between the two components. A comparison of nanofluid and deionized water data reveals that the temperature difference between the hot- and cold-side, ΔT = T_h ? T_c ≈ 0, is almost zero for nanofluid whereas ΔT > 0 for water. When ΔT ≈ 0, the contribution of Fourier effect to the overall heating is approximately zero hence enhancing the module cooling capacity. Experimental evidence further shows that temperature gradient across the thermal paste that bonds the chip and heat exchanger is much lower for the nanofluid than for deionized water. Low temperature gradient results in low resistance to the flow of heat across the thermal paste. The average thermal contact resistance, R = ΔT/Q, is 0.18 and 0.12 °C/W, respectively for the deionized water and nanofluid. For the range of optimum current, 1.2 ? current ? 4.1 A, considered in this study, the COP ranges between 1.96 and 0.68.     

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.     

Experimental study of R-134a evaporation heat transfer in a narrow annular duct

An experiment is carried out here to investigate the evaporation heat transfer and associated evaporating flow pattern for refrigerant R-134a flowing in a horizontal narrow annular duct. The gap of the duct is fixed at 1.0 and 2.0 mm. In the experiment, the effects of the duct gap, refrigerant vapor quality, mass flux and saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficient h_r are examined in detail. For the duct gap of 2.0 mm, the refrigerant mass flux G is varied from 300 to 500 kg/m² s, imposed heat flux q from 5 to 15 kW/m², vapor quality x_m from 0.05 to 0.95, and refrigerant saturation temperature T_sat from 5 to 15 °C. While for the gap of 1.0 mm, G is varied from 500 to 700 kg/m² s with the other parameters varied in the same ranges as that for δ = 2.0 mm. The experimental data clearly show that the evaporation heat transfer coefficient increases almost linearly with the vapor quality of the refrigerant and the increase is more significant at a higher G. Besides, the evaporation heat transfer coefficient also rises substantially at increasing q. Moreover, a significant increase in the evaporation heat transfer coefficient results for a rise in T_sat, but the effects are less pronounced in the narrower duct at a low imposed heat flux and a high refrigerant mass flux. Furthermore, the evaporation heat transfer coefficient increases substantially with the refrigerant mass flux except at low vapor quality. We also note that reducing the duct gap causes a significant increase in h_r. In addition to the heat transfer data, photos of R-134a evaporating flow taken from the duct side show the change of the dominant two-phase flow pattern in the duct with the experimental parameters. Finally, an empirical correlation for the present measured heat transfer coefficient for the R-134a evaporation in the narrow annular ducts is proposed.     

Mixed convection in a lid driven square cavity with an isothermally heated square blockage inside

Laminar mixed convection characteristics in a square cavity with an isothermally heated square blockage inside have been investigated numerically using the finite volume method of the ANSYS FLUENT commercial CFD code. Various different blockage sizes and concentric and eccentric placement of the blockage inside the cavity have been considered. The blockage is maintained at a hot temperature, T_h, and four surfaces of the cavity (including the lid) are maintained at a cold temperature, T_c, under all circumstances. The physical problem is represented mathematically by sets of governing conservation equations of mass, momentum, and energy. The geometrical and flow parameters for the problem are the blockage ratio (B), the blockage placement eccentricities (?_x and ?_y), the Reynolds number (Re), the Grashof number (Gr), and the Richardson number (Ri). The flow and heat transfer behavior in the cavity for a range of Richardson number (0.01–100) at a fixed Reynolds number (100) and Prandtl number (0.71) is examined comprehensively. The variations of the average and local Nusselt number at the blockage surface at various Richardson numbers for different blockage sizes and placement eccentricities are presented. From the analysis of the mixed convection process, it is found that for any size of the blockage placed anywhere in the cavity, the average Nusselt number does not change significantly with increasing Richardson number until it approaches the value of the order of 1 beyond which the average Nusselt number increases rapidly with the Richardson number. For the central placement of the blockage at any fixed Richardson number, the average Nusselt number decreases with increasing blockage ratio and reaches a minimum at around a blockage ratio of slightly larger than 1/2. For further increase of the blockage ratio, the average Nusselt number increases again and becomes independent of the Richardson number. The most preferable heat transfer (based on the average Nusselt number) is obtained when the blockage is placed around the top left and the bottom right corners of the cavity.     

A simulation study of heat and mass transfer in a honeycombed rotary desiccant dehumidifier

A one-dimensional coupled heat and mass transfer model, which is expected for use in designing and manufacturing of a honeycombed rotary desiccant wheel, is presented in this paper. The mathematical model has been validated using a real desiccant wheel, and the calculation results are in reasonable agreement with the experimental data. Based on this model, the temperature and humidity profiles in the wheel during both the dehumidification and the regeneration processes are analyzed and verified by experimental data. The numerical results indicate that in the regeneration process a hump curve of air humidity ratio along the channel exists all the time. In the regeneration process the hump of air humidity ratio moves from the duct entrance to the duct exit and increases gradually until the hump reaches the duct exit, where the hump will drop subsequently. The effects of velocity of regeneration air V_reg inlet temperature of regeneration air T_reg and velocity of process air V_ad on the hump moving speed are investigated. To improve the performance of desiccant wheel, it is essential to accelerate the hump moving from the duct entrance to the duct exit as soon as possible.     

Effect of nano-particles on forced convection in sinusoidal-wall channel

In this paper heat transfer and flow field in a wavy channel with nano-fluid is numerically studied. The temperature of input fluid (T_c) is taken less than that of the wavy horizontal walls (T_w). The governing equations are numerically solved in the domain by the control volume approach based on the SIMPLE technique. Copper–water nano-fluid is considered for simulation. A wide spectrum of numerical simulations has been done over a range of Reynolds number, Re_H, 5 ≤ Re_H ≤ 1500, nano-fluid volume fraction, ?, 0 ≤ ? ≤ 20% and the wave amplitude, α, 0 ≤ α ≤ 0.3. The effects of these parameters are investigated on the local and average Nusselt numbers and the skin friction coefficient. Simulations show excellent agreement with the literature. From this study, it is concluded that heat transfer in channels can enhance by addition of nano-particles, and usage of wavy horizontal walls. These can enhance the heat transfer by 50%. The present work can provide helpful guidelines to the manufacturers of the compact heat exchangers.