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.     

Study of the influence of the number of holes rows on the convective heat transfer in the case of full coverage film cooling: Etude de l’influence du nombre de rangées de trous sur les échanges convectifs dans le cas du refroidissement par multiperforation

The protection of the combustion wall chamber of aeronautic propeller is assumed with a full coverage film cooling. In the case of 1-9 rows of eight staggered holes, we have measured several profiles of temperature in the mainstream perturbed by jets. Variations of wall temperatures T_p have been measured simultaneously along the wall. Those experiences have been performed for three thermal cases, which are a function of the mainstream temperature T_e, the injected temperature T_i and the electrical density ?_dissi. imposed at the wall. Three blowing rates M=ρ_iU_i/ρ_eU_e are imposed too. The results are compared with the reference case without holes. The exam of the temperature profiles allows us to find a minimum number of rows to create a homogeneous cold layer. Two schemes of the convective heat transfer with two coefficients are presented and allow to represent each of the three thermal cases for the three blowing rates. Those models take into account of the three characteristic temperatures (T_e, T_i and T_p). One model is studied for 9 rows with infrared thermography. For five blowing rates, four zones of variations and an influence of M on the two coefficients are underlined.     

1/f noise and self-organized criticality in crisis regimes of heat and mass transfer

Thermal fluctuations under the transition from nucleate to film boiling of water on a wire heater and fluctuations of jet form during an outflow of superheated liquid from a pressure vessel were experimentally investigated. It has been found that power spectrum of the fluctuations has got a low frequency component corresponding to a 1/f law (flicker noise).The effect given is connected with the occurrence of nonequilibrium phase transitions in the systems: a crisis of heat transfer under the transition from nucleate to film boiling and a crisis of a flow in boiling-up of superheated liquid jet.     

Numerical investigation of the pool film boiling of water and R134a over a horizontal surface at near-critical conditions

Saturated pool film boiling over a flat horizontal surface is investigated numerically for water and refrigerant R134a at near-critical conditions for wall superheats (ΔT_Sup) of 2?K, 5?K, 8?K, 10?K, 15?K, and 20?K. The flow is considered to be laminar and incompressible. The governing equations are solved using a finite volume method with a collocated grid arrangement. For capturing the interface in two-phase boiling flows, a Coupled Level Set and Volume of Fluid (CLSVOF) with a multidirectional advection algorithm is used. Both single-mode and multimode boiling models are used for the numerical investigation to understand the effect of computational domain sizes on flow and heat transfer characteristics. In the case of water, the evolution of interface morphology shows the formation of a discrete periodic bubble release cycle occurring at lower Jacob numbers, Ja_v?≤?10.2(ΔT_Sup?≤?8?K), and the generation of jets of stable vapor film columns occurs at higher Ja_v?≥?12.7 (ΔT_Sup?≥?10?K). In the case of R134a, for all the Ja_v values considered in this study (0.163?≤?Ja_v?≤?1.63), the formation of a discrete periodic bubble release is observed. The results show that multimode boiling model should be used to understand the flow characteristics better. The magnitude of average Nusselt number obtained from the multimode film boiling model is lower than that of the single-mode film boiling model. The Nusselt numbers obtained from the present numerical studies are also compared with the available semiempirical correlations.     

Saturation boiling of HFE-7100 from a copper surface, simulating a microelectronic chip

Experiments are performed, which investigated the effect of inclination angle, θ, on saturation pool boiling of HFE-7100 dielectric liquid from a smooth, 10×10 mm copper surface, simulating a microelectronic chip. For θ?90° and surface superheats, ΔT_sat>20 K, nucleate boiling heat flux decreases with increased θ, but increases with θ for ΔT_sat<20 K. Similarly, at higher inclinations and ΔT_sat>13 K, nucleate boiling heat flux decreases with increased inclination, but at lower surface superheats the trend is inconclusive. The developed nucleate boiling correlation is within ±10% of the data and the developed correlations for critical heat flux (CHF) and the surface superheat at CHF are within ±3% and ±8% of the data, respectively. Results show that CHF decreases slowly from 24.45 W/cm² at 0° to 21 W/cm² at 90°, then decreases fast with increased θ to 4.30 W/cm² at 180°. The surface superheat at CHF also decreases with θ, from 31.7 K at 0° to 19.9 K at 180°. Still photographs are recorded of pool boiling at different heat fluxes and θ=0°, 30°, 60°, 90, 120°, 150° and 180°. The measured average departure bubble diameter from the photographs taken at the lowest nucleate boiling heat flux of ∼0.5 W/cm² and θ=0° is 0.55±0.07 mm and the calculated departure frequency is ∼100 Hz.     

Convection in a horizontal rectangular duct under constant and variable property formulations

The convective laminar flow through a horizontal rectangular duct, with significant buoyancy effects, has been investigated analytically and numerically, employing the constant property (CP) model and the variable property (VP) model. The duct has a finite heated region on the bottom and three-dimensional transport is studied. The CP model employs the Boussinesq approximations and uses properties evaluated at four different reference temperatures, i.e. the ambient temperature T₀, the average temperature T_f, the integrated average temperature T_int, and the heat source temperature T_h. In the VP model, the density and the transport properties are computed using the state equation of an ideal gas and power law correlations, respectively. Numerical results for temperature ratio ?, where ?=(T_h−T₀)/T₀, ranging from 0.033 to 2.33 are presented. The spanwise variation of the transport quantities is investigated in detail for the different models, and several interesting and important tends are obtained. These results will be of considerable value in model development for a wide variety of thermal systems and processes, where large changes in material properties are encountered.     

The role of global and detailed kinetics in the first-stage ignition delay in NTC-affected phenomena

Using n-heptane as a representative fuel exhibiting NTC (negative temperature coefficient) chemistry, a comprehensive computational and mechanism study was conducted on the role and controlling chemistry of the first-stage ignition delay in the superficially dissimilar systems of the auto-ignition of homogeneous mixtures and the nonpremixed counterflow ignition of fuel versus heated air. It is first shown that the first-stage auto-ignition delay time, τ₁, possesses a minimum value, τ_1,min, with increasing temperature, and that for temperatures below the range corresponding to τ_1,min, τ₁ is largely insensitive to the equivalence ratio (?) and pressure (p) of the mixture. Furthermore, in this regime the global reaction order was found to be close to unity, hence supporting the notion that the limiting steps in this temperature regime are the RO₂ isomerization reactions, which in turn explains the insensitivity of τ₁ on ? and p in this temperature regime. However, when the temperature approaches that of τ_1,min, competition of QOOH decomposition and the β scission reactions of the alkyl radicals with the low-temperature chemistry chain reactions, as well as the equilibrium shift of the oxygen addition reactions, increases τ₁ and consequently results in τ_1,min. The corresponding global reaction order also increases, to about two, indicating the progressive importance of the oxygen addition reactions. Extracted values of the global activation energy are also close to those of the controlling reactions in these temperature regimes. Results from the counterflow show the same global kinetic responses by identifying the reciprocal of the counterflow strain rate as the relevant ignition delay time and the temperature of the heated air stream as the homogeneous mixture temperature. It is further found that in the temperature range corresponding to τ_1,min, the diminished heat release causes the counterflow to lose its characteristic, non-monotonic S-curve response and consequently distinct, abrupt ignition–extinction transition events.     

Interplay between morphology and electrochemical performance of “core–shell” electrocatalysts for oxygen reduction reaction based on a PtNix carbon nitride “shell” and a pyrolyzed polyketone nanoball “core”

The interplay between morphology and electrochemical performance of a new class of “core–shell” electrocatalysts for the oxygen reduction reaction (ORR) is studied. The electrocatalysts, labelled PtNi–CN_lT_f/ST_p, consist of a “core” of pyrolyzed polyketone nanoballs (indicated as ST_p “core” support) covered by a carbon nitride (CN) “shell” matrix embedding PtNi_x alloy NPs (indicated as PtNi_x-CN). The electrocatalysts are characterized by means of: (a) high-resolution transmission electron microscopy (HR-TEM); and (b) cyclic voltammetry with the thin-film rotating ring-disk electrode (CV-TF-RRDE) method. The structure of the ST_p “core” supports and the details of the preparation procedure, such as pyrolysis temperature, T_f, and treatment with H₂O₂, play a crucial role on modulating: (a) the morphology; and (b) the ORR performance of the electrocatalysts. In particular, the best results are achieved for PtNi–CN_lT_f/ST_p systems: (a) including a ST_p “core” support with a high porosity; and (b) obtained at T_f = 600 °C. It is demonstrated that in general, the treatment with H₂O₂ of electrocatalysts is detrimental for the ORR performance. Nevertheless, in particular conditions, the treatment in H₂O₂ improves the ORR performance of PtNi-CN_lT_f/ST_p. The results presented in this work allow to elucidate the complex correlation existing between: (a) the composition; (b) the interactions in PtNi_x-CN; (c) the morphology of ST_p and PtNi–CN_lT_f/ST_p; and (d) the ORR performance of the electrocatalysts.     

Thermal protection with liquid film in turbulent mixed convection channel flows

In this numerical study, a channel flow of turbulent mixed convection of heat and mass transfer with film evaporation has been conducted. The turbulent hot air flows downward of the vertical channel and is cooled by the laminar liquid film on both sides of the channel with thermally insulated walls. The effect of gas–liquid phase coupling, variable thermophysical properties and film vaporization are considered in the analysis. In the air stream, the k–ε turbulent model has been utilized to formulate the turbulent flow. Parameters used in this study are the mass flow rate of the liquid film B, Reynolds number Re, and the free stream temperature of the hot air T_o. Results show that the heat flux was dramatically increases due to the evaporation of liquid water film. The heat transfer increases as the mass flow rate of the liquid film decreases, while the Reynolds number and inlet temperature increase, and the influences of the Re and T_o are more significant than that of the liquid flow rate. It is also found that liquid film helps lowering the heat and mass transfer rate from the hot gas in the turbulent channel, especially at the downstream.     

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.     

High heat flux removal using water subcooled flow boiling in a single-side heated circular channel

High heat flux removal from plasma-facing components and electronic heat sinks involves conjugate heat transfer analysis of the applicable substrate and flowing fluid. For the present case of subcooled flow boiling inside a single-side heated circular channel, the dimensional results show the significant radial, circumferential and axial variations in all thermal quantities for the present radial aspect ratio (R_o=outside radius to inside radius) of 3.0. A unified, dimensionless representation of the two-dimensional inside wall heat flux, and the dimensional inside wall heat flux (q_i(φ,z)) and temperature (T_i(φ,z)) data was found and used to collapse the data for all circumferential locations. Finally, 2-D boiling curves are presented and are among the first full set of 2-D boiling data presented for a single-side heated circular configuration.     

The numerical computation of the evaporative cooling of falling water film in turbulent mixed convection inside a vertical tube

An analysis has been developed for studying the evaporative cooling of liquid film falling inside a vertical insulated tube in turbulent gas stream is presented. Heat and mass transfer characteristics in air–water system are mainly considered. A low Reynolds number turbulence model of Launder and Sharma is used to simulate the turbulent gas stream and a modified Van Driest model suggested by Yih and Liu is adopted to simulate the turbulent liquid film. The model predictions are first compared with available experimental data for the purpose of validating the model. Parametric computations were performed to investigate the effects of Reynolds number, inlet liquid temperature and inlet liquid mass flow rate on the liquid film cooling mechanism. Results show that significant liquid cooling results for the system with a higher gas flow Reynolds number Re, a lower liquid flow rate Γ₀ or a higher inlet liquid temperature T_L0.     

Kinetic model and prediction for coal hydrogasification

Coal hydrogasification is a key component of zero emission coal (ZEC) power generation system which discharges little CO₂ and other pollutants at a thermal efficiency close to 70%. In addition, coal hydrogasification itself has many advantages. A hydrogasification kinetic model including ten homogeneous reactions and four heterogeneous reactions is established in this work and is validated against experiment data available in literatures. The validated model is then used to predict the effects of different reaction conditions including the reaction temperature T, the reaction pressure p_t, the H₂/coal mass ratio U and the reaction time t on coal hydrogasification properties. The results indicate that coal hydrogasification is facilitated by the increased p_t and t. When T is not higher than 1273 K, the gasification process is promoted with T increment. Increasing U can promote the coal hydrogasification process on the whole. When U is larger than 0.5, however, the coal conversion ratio (x_coal) will slightly decrease with U increment.     

High-pressure hydrogen/carbon monoxide syngas turbulent burning velocities measured at constant turbulent Reynolds numbers

Using a double-chamber explosion facility, we measure high-pressure turbulent burning velocities (S_T) of lean syngas (35%H₂/65%CO) spherical flames at constant turbulent Reynolds numbers (Re_T ≡ u′L_I/ν) varying from 6700 to 14,200, where the root-mean-square turbulent fluctuation velocity (u′) and the integral length scale (L_I) are adjusted in proportion to the decreasing kinematic viscosity of reactants (ν) at elevated pressure (p) up to 1.2 MPa. Results show that, contrary to popular scenario for turbulent flames, at constant Re_T, S_T decreases similarly as laminar burning velocities (S_L) with increasing p in minus exponential manners. Moreover, at constant p, S_T/S_L increases noticeably with increasing Re_T. It is found that the present very scattering S_T/S_L data at different p and Re_T can be nicely merged onto a relation of S_T/u′ = 0.49Da^0.25, where Da is the turbulent Damköhler number and values of S_T/u′ tends to level-off when Da > 160 and p > 0.7 MPa.     

Model for boiling explosion during rapid liquid heating

The process of rapid liquid heating with a linearly increasing boundary temperature condition has been simulated by applying the analytical solution of 1D semi-infinite heat conduction in association with the molecular theory of homogeneous nucleation boiling. A control volume having the size of a characteristic critical cluster at the liquid boundary is considered, and the corresponding energy balance equation is obtained by considering two parallel competing processes that take place inside the control volume, namely, transient external energy deposition and internal energy consumption due to bubble nucleation and subsequent growth. Depending on the instantaneous rate of external energy deposition and boiling heat consumption within the control volume, a particular state is defined as the boiling explosion condition in which bubble generation and growth cause the liquid sensible energy to decrease. The obtained results are presented in terms of the average liquid temperature rise within the control volume, maximum attainable liquid temperature before boiling explosion and the time required to achieve the condition of boiling explosion. The model is applied for the case of water heating at atmospheric pressure with initial and boundary conditions identical to those reported in the literature. Model predictions concerning boiling explosion are found to be in good agreement with the experimental observations. The boiling explosion condition as predicted by the present model is verified by comparing the heat flux across the liquid–vapor interface with the corresponding limit of maximum possible heat flux, q_max,max, at the time of boiling explosion. A comparative study between the actual heat flux and the limit of maximum heat flux, q_max,max, at the time of boiling explosion for different rates of boundary heating indicates that, with much higher boundary heating rates, it is possible to heat the liquid to a much higher temperature before theoretical instantaneous boiling explosion occurs.     

Explosive boiling of liquid argon films on flat and nanostructured surfaces

Abstract

Because of the effects of the nanostructure, phase change behaviors on flat and nanostructured surfaces display distinct features. In this work, the molecular dynamics simulation method is employed to investigate the onset temperature of explosive boiling (T_s) with various film thicknesses, pillar heights, and wettability. The simulation results show that T_s decreases with the film thickness on both wettability flat surfaces. However, the decreasing rates have the significant distinctions, where the difference between two surfaces of T_s with the identical film thickness decreases. In addition, the simulation results demonstrate that all the values of T_s on nanostructured surfaces are lower than those on flat surfaces with the same film thickness. With the increase of the film thickness, T_s presents a decreasing trend on both wettability and nanostructured surfaces, especially with the liquid film with the thickness over 6?nm, where a completely opposite conclusion compared to the flat surface is represented.     

Measuring temperature of the ice surface during its formation by using infrared instrumentation

A non-destructive remote sensing technique was used to measure the surface temperature of a thin macroscopic water film flowing on a growing asymmetric ice accretion during its formation inside an icing research wind tunnel. Given the underlying thermodynamic conditions of this experimental series, the recorded surface temperature was always below the temperature of water fusion, T_m = 273.15 K, even when water shedding from growing ice accretions was observed visually. The surface temperature of ice accretions, T_s, ranged from −1 °C, for angular positions near the stagnation line, down to a certain minimum above the ambient temperature, T_a, for the greater angular positions, i.e. T_m > T_s > T_a.     

The influence of mass transport on the heat transfer coefficients during the boiling of multicomponent mixtures

The paper describes a mathematical model of the process based on multicomponent mass transfer theory which enables the effect to be predicted of mass transport on the boiling heat transfer coefficient. The results of calculations were compared with our own experimental pool boiling data for the ternary system methanol–isopropanol–water and with Grigoriev's data obtained for the system acetone–methanol–water. The good accuracy was obtained when the ratio of the tube surface area to the surface area of the bubbles, which touch the heater at that moment, was considered as a parameter of the model. Based on our own experimental pool boiling data for the system methanol–isopropanol–water and the corresponding binary systems the triangular diagram of the ratio α_exp/α_id as a function of the liquid compositions is presented.