جستجوی مقالات مرتبط با کلیدواژه « heat flux » در نشریات گروه « مکانیک »

Homogeneous charge compression ignition (HCCI) has attracted lots of attention due to the high thermal efficiency, lower NO x, and Soot exhaust emissions. Heat transfer from the gases to the combustion chamber walls has effective impact on the combustion process and the formation of engine-out emissions in HCCI engine. In this study for the first time a zero-dimensional single-zone model coupled with detailed chemical kinetics was used to evaluate the semi-empirical heat transfer models of Annand, Woschni, Hohenberg along with Assanis and Hensel to calculate heat flux in an HCCI engine fueled with natural gas. For this purpose, the 3D-CFD model coupled with detailed chemical kinetics was firstly validated by using experimental data, and then the 3D derived model was used as a base model for evaluating zero-dimensional model. Furthermore, the response surface model (RSM) was employed for investigating the effect of input parameters of engine including intake pressure (1, 1.25, and 1.5bar), equivalence ratio (0.3, 0.5, and 0.7), and engine speed (800, 1100, and 1400rpm) on the output parameters i.e., in-cylinder pressure and heat flux. In most cases were assessed, the zero-dimensional simulation results indicated that Annand technique provided the best model for heat flux simulation. Besides, the model of Hohenberg overpredict the heat flux in comparison with the calculated values derived from the 3D model, while Assanis and Hensel models underpredict the heat flux compared with the evaluated value of the 3D model. Furthermore, Woschni’s model cannot be used to model the heat flux in HCCI engine.

In the present study, the nucleate and film boiling heat transfer of alumina-deionized water and silica-deionized water nanofluids in the presence of microchannel surface has been investigated experimentally. For this purpose, both types of nanofluids with volumetric concentrations of 0.1%, 0.3% and 0.5% were prepared using two-step method. Pure deionized water boiling experiments on polished and microchannel copper surfaces and nanofluid boiling experiments on microchannel copper surface were performed. The results showed that the combination of the presence of nanofluid and microchannel in the boiling process significantly increased the critical heat flux and the minimum heat flux. In the boiling of nanofluids on microchannel surface, the volumetric concentration of 0.5% had the best performance, so that the maximum values of critical heat flux and minimum heat flux for alumina-deionized water nanofluid increased by 93.02% and 105.32%, and for silica-deionized water nanofluid increased by 90.82% and 97.15%, respectively, compared to pure deionized water boiling on the polished surface. In addition, it was observed that with the deposition of nanoparticles on the microchannel surface, the surface wettability increased and the nucleate boiling heat transfer coefficient of nanofluids decreased compared to pure deionized water. But in film boiling, the effects of improving the thermal properties of nanofluids have overcome the destructive effects of nanoparticles deposition on the surface.

This paper dealing with a novel algorithm based on features of Fuzzy-PID controllers to estimate heat flux on the inverse heat conduction problems. The main structure of Fuzzy-PID is a PID controller in which the proportionalو integrator and the derivative gains are obtained online by fuzzy system. The input of the algorithm is the measured temperatures within the model. In each time-step, the smart controller calculates the proper heat flux in order to adjust the measured temperature with desired input temperature. The model studied a flat plate with an insulated surface and an active level that affects the variable heat flux at the time. The variation of heat flux with time can be considered to be constant, step, and triangular. The measured temperatures are obtained at the active and inactive surface with numerical simulation. The effect of noise level at the measurement temperatures on the accuracy of the proposed method is investigated. The estimations and error analysis indicate that this algorithm is very successful in estimating the different forms of heat flux with different amounts of noise and the different thermocouple positions in the wall.

Temperature sensors have recently been proposed for attitude estimation (AE) of Low-Earth satellites. However, since half of the satellite surfaces do not receive any heat flux from the Sun, conduction occurs among the satellite surfaces. In this regard, the present study has focused on the effect of surfaces’ conduction as well as inner radiation on AE using temperature sensors. The nonlinear filter of Unsceted Kalman filter is adopted for AE, and the developed model to describe temperature rates is verified using Thermal Desktop and SINDA software. Monte Carlo simulations prove positive effect of the conduction on AE performance against negative role of the inner radiation.

In the present study, with the help of numerical study, the study of the uniform heat flux formation on process tubes as the main parameter in cracking furnaces has been investigated using flat flame burners with different thermal powers. Due to the lack of experimental data for solver validation, two problems of swirl burner and channel with conjugate heat transfer of combustion gases and solid surface have been used. To carry out simulations, the chtMultiRegionReactingFOAM solver in OpenFOAM software has been developed by adding the conjugate heat transfer capability to the ReactingFOAM solver. In simulations, k-ω SST turbulence model has been used for turbulence modeling. The results of the simulations show that the use of a flat flame burner in cracking furnaces allows for the uniform temperature distribution in the furnace with the low maximum combustion temperature. Also, to create the appropriate heat flux around the pipes so that the proper temperature distribution for cracking reactions is provided, the minimum heat flux of the flat flame burners is required, which in less than that, the appropriate temperature distribution does not occur on the pipes.

In this paper presents a digital filter method for online estimation of heat flux on a one - dimensional plate. Two inverse problems are designed to investigate this method. In the first problem, the unknown heat flux and the boundary condition on the other side of the plate are not known and in the second problem, the heat flux on the sides of a two-layers plate is unknown. in both cases, two sensors have been used to measure the temperature history of the plate. In the proposed algorithm, one of the two sensors is used as a boundary condition. To evaluate the accuracy and capability of digital filter, several experiments were designed. The filter coefficients are calculated only once in the proposed method and used in all experiments. The results show that this algorithm is very successful in estimating different forms of heat flux with different noise intensity at measured temperatures and different thermocouple positions in the plate and the Tikhonov digital filter method has more precision compared to Beck’s method. Also, using the mollified data in the proposed algorithm increases the accuracy and efficiency of the proposed algorithm.

In this paper, entropy generated during a continuous two-dimensional boundary layer flow on a porous flat plate surrounded by avariable heat flux is studied through a similarity solution. This study is presented for modeling of the cooling process of metalsurfaces with porous coating that is widely used in industry. Using similarity variables, the continuity, momentum,energyequations are transformed into ordinary differential equations. To validate the proposed solution, results are compared to the existingstudies. The effects of various parameters such as heat flux (q􀭵), suction parameter (S), Prandtl number (Pr), Stretch parameter onthe plate velocity (U), temperature distribution (T),dimensionless entropy (Ns) are shown,discussed in detail. Bijan numberis used as an important parameter for qualitative study in cooling process on porous surfaces. The increase of Bijan number isintroduced as a measure for the increase of cooling rate. The results of the study show that when Prandtl number increases, theentropy will decrease which leads to a decrease in Bijan number. However, an increase in permeability of surface causes toincreasing suction parameter,Bijan number.

Heat transfer enhancement is widely applicable in various industries, specifically in heat exchangers. Optimizing of heat transfer in the absence of increased pumping energy will result in increased of total efficiency in different systems. In this paper, forced convection heat transfer and fluid flow of fully developed laminar regime in a horizontal tube under uniform and non-uniform step heat fluxes is investigated experimentally. The effect of uniform, non-uniform increasing and decreasing applied heat fluxes on heat transfer and fluid flow are investigated. The effect of various parameters on heat transfer and fluid flow characteristics in these models are reported. Uncertainty analysis is performed and acceptable maximum of 1.8 percent is acquired. The primary results compared to well-known Shah and London equation for validation and maximum error of 8.5 percent is reported. In the present paper, Energy and exergy are two approach of analyzing. Convection heat transfer coefficient enhancement of 19.3 and 22.3 percent compared with model 1 are reported for model 2 and 3 respectively, in energy analysis. Furthermore, in this paper, exergy analysis is done and irreversibility values of 0.0887, 0.0803 and 0.1037 are reported for model 1, model 2 and model 3 respectively. Finally, it is concluded that the model number 3 is the best way to enhance heat transfer because of the maximum averaged Nusselt number and the minimum entropy generation values

This study implements an inverse method for obtaining heat input during the friction welding process in terms of time, with specified sequences. Using limited experimental data leads to precise evaluation of heat generation during the process. Solution accuracy was improved by integration of analytical formulation, consideration of temperature-dependent thermophysical properties and slip definition in three-dimensional finite element model. Specifically, conjugate gradient method was used as inverse method to calculate the unknown parameters. Heat generation within sliding, sticking state was obtained using friction and slip dependence of time and deducted from exponential approximation of angular velocity of matrix. Favorable situation for friction stir welding is the case that workpiece does not experience the melting temperature. Online CO2 gas cooling flow was used to obligate the desired span of temperature. Numerical calculation of heat flux during friction stir welding of Al5052-O accompanied by cooling stage was considered and inverse method was integrated to evaluate governing parameters. Effect of cooling stage in temperature distribution of workpiece was simulated numerically. Calculated unknown parameters indicate good prediction of heat flux, which satisfy the measured temperature, and characteristic parameters of the process.

In this research, the electrothermal characteristics of anode have been analytically investigated by considering two subzones including the quasi-neutral and non-neutral plasma flow near the anode surface. A correlation between current density and anode fall voltage has been derived by using the continuity and momentum equations for electron in the non-neutral plasma subzone. Also, two relations have been deduced from the magnetic field induction equation and Ohm’s law to determine the current densities and voltage drops in the quasi-neutral plasma subzone. The suggested solution algorithm has been based on the satisfaction of current continuity condition to be converged by a trial and error method. The Princeton benchmark thruster has been investigated for the discharge current of 8 kA with mass flow rates of 4 and 24 gr/s to evaluate the developed algorithm. The obtained analytical results show that under operating conditions with mass flow rates of 4 and 24 g/s, the heat flux values are respectively in the range of 480 to1350 and 400 to1490 W/cm2, the current densities vary from 24 to 90 and from 33 to 140 A/cm2, and also the anode falls change from 5.8 to 11 and 4.9 to 6 V. The analytical algorithm has predicted the heat fluxes, current densities, and anode falls along the anode face in good agreement with measurements. It has been seen that the heat flux and current density around the anode mid-lip, where the anode fall has its minimum value, have been maximized. By using the second thermodynamic law, it has been shown that the probability density function of effective velocity has a Gaussian distribution. Therefore, under this circumstance, entropy has been maximized, and consequently the heat flux and current density profiles have been taken the summit form.

Approximately one third of fuel heat release of an IC engine is delivered to environment by cooling system. Cooling more than a limit, decreases efficiency of engine and lower than a limit, causes vapor layer formation on the surface of water jacket and reduction of heat transfer and failure components. Behavior of heat transfer coefficient in boiling depends on heat flux. This coefficient increases during nucleate boiling and decreases during film boiling. In this research, an experimental setup was designed to investigate the effect of heat flux on heat transfer from coolant. This study carried out with two base fluid, distilled water and mixture of distilled water and ethylene-glycol (50-50) and measurement of surface temperature. With consider that boiling flow takes place on hot spot of water jacket with low speed of coolant, therefore, velocity range in exhaust valve bridge has considered. Effect of inlet velocity and coolant temperature in constant heat flux on surface temperature and heat transfer coefficient, has been investigated. Up to experimental results, numerical simulation carried out with CFD software. The numerical results have good agreement with experimental data.

In this study, the aerodynamic heating of the flying body during powered flight phase has been numerically investigated. The conjugate simulation of fluid heat transfer and solid heat conduction has been considered. To this aim, the coupling boundary condition has been used for body shell that allows the conjugate heat transfer investigation in the fluid and solid domains simultaneously. The model has been considered as a circular cylinder and spherically blunted cone nose with 350mm in diameter. The investigation has been carried out at different Mach number from 1.5 to 4.2 to cover range of supersonic flow. The advantage of this method is that the wall temperature and heat flux ​​in any part of the nose and body shell with or without axial symmetry, connected components and other protuberances could be calculated at different angles of attack. Finally, the approach has been validated through the results of analytical and numerical methods for aerodynamic heating of axisymmetric vehicles.

The subcooled flow boiling occurs when the bulk temperature is less than saturation temperature of the liquid at that pressure while the surface temperature is higher. The most importance of boiling phenomenon is related to the high latent heat of fluid which could removes high heat flux at relatively low temperature difference between liquid and the hot surface. In this study, the impact of velocity and roughness on the subcooled flow boiling were investigated experimentally for pure water. An experimental setup was designed and manufactured. The experimental setup consists of a plexiglass channel with cross section 20×30 mm and the length of 120 cm. A cylindrical heater with diameter 12 mm made of copper is located on the bottom surface of the plexiglass channel. All the experiments were conducted for the surface roughness of 0.65, 2.5 and 4.4 µm at velocities of 0.5, 0.7 and 0.9 m/s. The experimental results show that the surface heat flux increases as the surface roughness and velocity increases. However, this affect of velocity enhancement was only observed for lower boiling surface temperature and opposite trend has taken place for higher boiling surface temperature. This is due to the simultaneous consideration of the convection and boiling terms along with the interaction between them which has not been presented experimentally yet. It appears that this kind of experimental study has not been carried out for copper type surfaces.