Showing papers in "Heat Transfer Engineering in 2010"
TL;DR: In this article, a literature survey of blade tip leakage flow and heat transfer, as well as research of external and internal cooling technologies, is presented, where the tip clearance gap is defined as the point where the complex tip leakage flows and local high heat loads prevail.
Abstract: Gas turbines are widely used for aircraft propulsion, land-base power generation, and other industrial applications like trains, marines, automobiles, etc. To satisfy the fast development of advanced gas turbines, the operating temperature must be increased to improve the thermal efficiency and output work of the gas turbine engine. However, the heat transferred to the turbine blade is substantially increased as the turbine inlet temperature is continuously increased. Thus, it is very important to cool the turbine blades for a long durability and safe operation. Cooling the blade must include cooling of the key regions being exposed to the hot gas. The blade tip region is such a critical area and is indeed difficult to cool. This results from the tip clearance gap where the complex tip leakage flow occurs and thereby local high heat loads prevail. This paper presents a literature survey of blade tip leakage flow and heat transfer, as well as research of external and internal cooling technologies. The pres...
100 citations
TL;DR: In this article, a general review of developments and improvements on helixchangers is provided, which includes discontinuous helical baffles, continuous or combined helical bifolds, and the combined multiple shell-pass helix changers.
Abstract: Helical baffles are employed increasingly in shell-and-tube heat exchangers (helixchangers) for their significant advantages in reducing pressure drop, vibration, and fouling while maintaining a higher heat transfer performance. In order to make good use of helical baffles, serial improvements have been made by many researchers. In this paper, a general review is provided of developments and improvements on helixchangers, which includes the discontinuous helical baffles, continuous or combined helical baffles, and the combined multiple shell-pass helixchangers. Extensive results from experiments and numerical simulations indicate that these helixchangers have better flow and heat transfer performance than the conventional segmental baffled heat exchangers. Based on these new improvements, the conventional heat exchangers with segmental baffles might be replaced by helixchangers in industrial applications to save energy, reduce cost, and prolong the service life and operation time.
93 citations
TL;DR: In this article, the authors explored the underlying heat transfer mechanisms and illustrates the similarities and differences between the two processes and showed that the formation of elongated bubbles and their passage over the microchannel walls have similarities to the bubble ebullition cycle in pool boiling.
Abstract: Recent literature indicates that under certain conditions the heat transfer coefficient during flow boiling in microchannels is quite similar to that under pool boiling conditions. This is rather unexpected, as microchannels are believed to provide significant heat transfer enhancement under single-phase as well as flow boiling conditions. This article explores the underlying heat transfer mechanisms and illustrates the similarities and differences between the two processes. Formation of elongated bubbles and their passage over the microchannel walls have similarities to the bubble ebullition cycle in pool boiling. During the passage of elongated bubbles, the longer duration between two successive liquid slugs leads to wall dryout and a critical heat flux that may be lower than that under pool boiling conditions. A clear understanding of these phenomena will help in overcoming these limiting factors and in developing strategies for enhancing heat transfer during flow boiling in microchannels.
73 citations
TL;DR: In this article, the authors analyzed some of the most common heat transfer applications at total sites comprising users of the types just mentioned, and optimized the energy demands, the local generation capacities, and the efficient integration of renewables into the corresponding total site CHP (combined heat and power) energy systems, based on efficient heat transfer, are optimized, minimizing heat waste and carbon footprint, and maximizing economic viability.
Abstract: The majority of industrial, residential, service, and business customers, as well as agriculture farms, are still dominated by fossil fuels as primary energy sources. They are mostly equipped with steam and/or gas turbines, steam boilers, and water heaters (running on electricity or gas) for conversion units. The challenge to increase the share of renewables in the primary energy mix could be met by integrating solar, wind, and biomass as well as some types of waste with the fossil fuels. This work analyzes some of the most common heat transfer applications at total sites comprising users of the types just mentioned. The energy demands, the local generation capacities, and the efficient integration of renewables into the corresponding total site CHP (combined heat and power) energy systems, based on efficient heat transfer, are optimized, minimizing heat waste and carbon footprint, and maximizing economic viability.
68 citations
TL;DR: In this paper, a closed-loop pulsating heat pipe (CLPHP) was designed and manufactured, and the effect of various parameters, including the working fluid (water and ethanol), the volumetric filling ratio (30, 40, 50, 70, 80%), and the input heat power (5 to 70 W), on the thermal performance of the CLPHP was investigated experimentally.
Abstract: Pulsating heat pipes are complex heat transfer devices, and their optimum thermal performance is largely dependent on different parameters. In this paper, in order to investigate these parameters, first a closed-loop pulsating heat pipe (CLPHP) was designed and manufactured. The CLPHP was made of copper tubes with internal diameters of 1.8 mm. The lengths of the evaporator, adiabatic, and condenser sections were 60, 150, and 60 mm, respectively. Afterward, the effect of various parameters, including the working fluid (water and ethanol), the volumetric filling ratio (30%, 40%, 50%, 70%, 80%), and the input heat power (5 to 70 W), on the thermal performance of the CLPHP was investigated experimentally. The results showed that the manufactured CLPHP has the best thermal performance for water and ethanol as working fluids when the corresponding filling ratios are 40% and 50%, respectively. Finally, with the available experimental data set of CLPHPs, a power-law correlation based on dimensionless groups was e...
60 citations
TL;DR: In this article, a systematic and careful experimental study of the friction factor in the transition region for single-phase water flow in mini-and microtubes has been performed for 12 stainless-steel tubes with diameters ranging from 2083 μ m to 337 μ m.
Abstract: A systematic and careful experimental study of the friction factor in the transition region for single-phase water flow in mini- and microtubes has been performed for 12 stainless-steel tubes with diameters ranging from 2083 μ m to 337 μ m. The pressure drop measurements were carefully performed by paying particular attention to the sensitivity of the pressure-sensing diaphragms used in the pressure transducer. Experimental results indicated that the start and end of the transition region were influenced by the tube diameter. The friction factor profile was not significantly affected for the tube diameters between 2083 μ m and 1372 μm. However, the influence of the tube diameter on the friction factor profile became noticeable as the diameter decreased from 1372 μ m to 337 μm. The Reynolds number range for transition flow became narrower with decreasing tube diameter.
55 citations
TL;DR: In this paper, the authors proposed nucleate boiling as the dominant micro-scale mechanism, based on the strong dependence of the heat transfer coefficient on the heat flux similar to nucleate pool boiling.
Abstract: Numerous characteristic trends and effects have been observed in published studies on two-phase micro-channel boiling heat transfer. While macro-scale flow boiling heat transfer may be decomposed into nucleate and convective boiling contributions, at the micro-scale the extent of these two important mechanisms remains unclear. Although many experimental studies have proposed nucleate boiling as the dominant micro-scale mechanism, based on the strong dependence of the heat transfer coefficient on the heat flux similar to nucleate pool boiling, they fall short when it comes to actual physical proof. A strong presence of nucleate boiling is reasonably associated to a flow of bubbles with sizes ranging from the microscopic scale to the magnitude of the channel diameter. The bubbly flow pattern, which adapts well to this description, is observed, however, only over an extremely limited range of low vapor qualities (typically for quality less than 0.01–0.05). Furthermore, at intermediate and high vapor qualitie...
45 citations
TL;DR: In this article, the effect of thermal pretreatment at temperatures around 160-175°C on sludge solubilization and digestion was investigated, and it was found that thermal pretreatments at around 160°C tend to produce better solubiliization.
Abstract: Sludge digestion and stabilization governs almost half of the operating cost of wastewater treatment plants, and hydrolysis is the limiting step in sludge digestion. Increasing hydrolysis by implementing pretreatment prior to digestion can increase the digestion efficiency. Among pretreatment methods, thermal pretreatment at temperatures around 160–175°C tends to produce better solubilization. Microwave (MW) pretreatment was recently studied as an alternative method to conventional thermal pretreatment. Until now, MW pretreatment above boiling point had not been studied for sludge solubilization and digestion. This paper provides preliminary results on the effect of MW pretreatment operating conditions such as high temperature (T) (110–175°C), MW intensity (I) (1.25 and 3.75°C/min), and sludge concentration (C) (6 and 11.85%) on solubilization. From experimental results and three-factor fixed-effect analysis of variance (ANOVA) determination it was found that T, I, and C main effects and T:I and T:C inter...
45 citations
TL;DR: In this article, an analytical study on forced convection of laminar fully developed flow of incompressible, constant-property nanofluids in microchannels is presented.
Abstract: This article presents an analytical study on forced convection of laminar fully developed flow of incompressible, constant-property nanofluids in microchannels. Closed-form solutions for the temperature distributions in the radial direction with the incorporation of viscous dissipation are obtained under isoflux boundary condition. The effects of the governing parameters, including modified Brinkman number, thermal conductivity ratio, and nanoparticle volume fraction of the nanofluids, on the temperature distributions are investigated and analyzed for both heating and cooling processes. The heat transfer performance characterized by the Nusselt number is investigated based on the effects induced by these parameters. In the comparison between the models with and without viscous dissipation, it is found that the thermal performance of a microchannel is overrated when viscous dissipation is excluded in the analysis. It is concluded that these governing parameters are intimately interrelated in the flow and t...
44 citations
TL;DR: In this article, a long-term study of flow boiling of R134a in five vertical stainless-steel tubes of internal diameter 4.26 mm down to 1.52 mm is presented.
Abstract: An overview of the recent developments in the study of flow patterns and boiling heat transfer in small to micro diameter tubes is presented. The latest results of a long-term study of flow boiling of R134a in five vertical stainless-steel tubes of internal diameter 4.26, 2.88, 2.01, 1.1, and 0.52 mm are then discussed. During these experiments, the mass flux was varied from 100 to 700 kg/m2s and the heat flux from as low as 1.6 to 135 kW/m2. Five different pressures were studied, namely, 6, 8, 10, 12, and 14 bar. The flow regimes were observed at a glass section located directly at the exit of the heated test section. The range of diameters was chosen to investigate thresholds for macro, small, or micro tube characteristics. The heat transfer coefficients in tubes ranging from 4.26 mm down to 1.1 mm increased with heat flux and system pressure, but did not change with vapor quality for low quality values. At higher quality, the heat transfer coefficients decreased with increasing quality, indicating loca...
42 citations
TL;DR: In this article, a concise survey of available methods for prediction of heat transfer coefficient in saturated boiling regime is given and in that light a modified author's own model is presented, contrary to other approaches, finds application in the cases of both conventional and small-diameter channels.
Abstract: Presented in this article are considerations regarding modeling of flow boiling in conventional, small-diameter channels and minichannels. A concise survey of available methods for prediction of heat transfer coefficient in saturated boiling regime is given and in that light a modified author's own model is presented. The presented model, contrary to other approaches, finds application in the cases of both conventional and small-diameter channels. The results of calculations are compared with some experimental data available from literature on conventional-size tubes and also minichannels. Obtained agreement is satisfactory.
TL;DR: In this article, a Bayesian approach was used for the simultaneous identification of volumetric heat capacity, thermal conductivity, and boundary heat flux, in a one-dimensional nonlinear heat conduction problem.
Abstract: In this article, we apply a Bayesian approach for the simultaneous identification of volumetric heat capacity, thermal conductivity, and boundary heat flux, in a one-dimensional nonlinear heat conduction problem. The Markov chain Monte Carlo sampling approach, implemented in the form of the Metropolis–Hastings algorithm, was used for the solution of the inverse problem. Simulated temperature measurements were used in the inverse analysis in order to examine the accuracy and stability of the overall approach. Independent measurement data were used to construct the prior model for the coefficients to be estimated. The approach is also applied to experiments involving the heating of a reference material with an oxyacetylene torch.
TL;DR: In this paper, the influence of the thermal physics of nanofluids on the critical heat flux (CHF) of a pool boiling experiment with electrically heated NiCr metal wire under atmospheric pressure was investigated.
Abstract: This study investigates the influence of the thermal physics of nanofluids on the critical heat flux (CHF) of nanofluids. Thermal physics tests of nanoparticle concentrations ranged from 0 to 1 g/L. Pool boiling experiments were performed using electrically heated NiCr metal wire under atmospheric pressure. The results show that there was no obvious change for viscosity and a maximum enhancement of about 5 to 7% for thermal conductivity and surface tension with the addition of nanoparticles into pure water. Consistently with other nanofluid studies, this study found that a significant enhancement in CHF could be achieved at modest nanoparticle concentrations (<0.1 g/L by Al2O3 nanoparticle concentration). Compared to the CHF of pure water, an enhancement of 113% over that of nanofluids was found. Scanning electron microscope photos showed there was a nanoparticle layer formed on the heating surface for nanofluid boiling. The bubble growth was photographed by a camera. The coating layer makes the nucleatio...
TL;DR: The validity and limitations of the numerous two-phase non-boiling heat transfer correlations that have been published in the literature over the past 50 years are discussed in this article, and the application of these correlations in engineering practice and how they can influence the equipment design and consequently the process design are discussed.
Abstract: The validity and limitations of the numerous two-phase non-boiling heat transfer correlations that have been published in the literature over the past 50 years are discussed. The extensive results of the recent developments in the non-boiling two-phase heat transfer in air–water flow in horizontal and inclined pipes conducted at Oklahoma State University's two-phase flow heat transfer laboratory are presented. Practical heat transfer correlations for a variety of gas–liquid flow patterns and pipe inclination angles are recommended. The application of these correlations in engineering practice and how they can influence the equipment design and consequently the process design are discussed.
TL;DR: In this paper, the effect of hole trenching on the cooling performance of a high-pressure turbine blade near the leading edge was investigated and the main effect was the reduction of jet lifting off from the blade surface and so the prevention of sudden lowering of cooling effectiveness after the injection location.
Abstract: Computational results are presented for a row of coolant injection holes on each side of a high-pressure turbine blade near the leading edge. Seven hole configurations have been used to show the effect of various diffusion shaped holes and their trenching on film cooling effectiveness: (1) cylindrical film hole; (2) forward diffused film hole; (3) trenched forward diffused film hole; (4) conically flared film hole; (5) trenched conically flared film hole; (6) laterally diffused film hole; and (7) trenched laterally diffused film hole. Computational solutions of the steady, Reynolds-averaged Navier–Stokes equations are obtained using a finite-volume method. Results show that the main effect of trenching is the reduction of jet lifting off from the blade surface and so the prevention of sudden lowering of cooling effectiveness after the injection location. Moreover, hole trenching has more effect on film cooling flow on the suction side than on the pressure side. Also, the trenched laterally diffused shaped...
TL;DR: In this article, a novel heat exchanger/reactor is characterized in terms of residence time, pressure drops, and thermal behavior in order to estimate its capacities to perform an exothermic reaction: the oxidation of sodium thiosulfate by hydrogen peroxide.
Abstract: The implementation of chemical syntheses in a batch or semi-batch reactor is generally limited by the removal or the supply of heat. A way to enhance thermal performances is to develop multifunctional devices like heat exchanger/reactors. In this work, a novel heat exchanger/reactor is characterized in terms of residence time, pressure drops, and thermal behavior in order to estimate its capacities to perform an exothermic reaction: the oxidation of sodium thiosulfate by hydrogen peroxide. Experimental results highlight the performances of the heat exchanger/reactor in terms of intensification, which allows the implementation of the oxidation reaction at extreme operating conditions. These conditions are finally compared to the ones of a classical batch reactor.
TL;DR: In this article, the effect of a non-adsorbable gas (air) on the kinetics of water adsorption on loose grains was studied for three adsorbents promising for adaption chilling: SWS-1L (silica KSK modified by calcium chloride), silica Fuji type RD and FAM-Z02.
Abstract: In this article the effect of a non-adsorbable gas (air) on kinetics of water adsorption on loose grains was studied for three adsorbents promising for adsorption chilling: SWS-1L (silica KSK modified by calcium chloride), silica Fuji type RD, and FAM-Z02. The experimental conditions were fixed similar to real operating conditions during an isobaric adsorption stage of the basic cycle of an adsorption chiller (AC). Reduction of the adsorption rate was revealed even at a partial pressure of residual air as low as 0.06 mbar. Dependence of the characteristic adsorption time on air partial pressure was found to be linear for partial pressures greater than 0.4 mbar, with the slope depending on the adsorbent nature. Desorption stage was less affected by the residual air. Specific power released in an AC evaporator during the adsorption process was estimated as a function of the partial pressure of residual air, and recommendations how to improve cooling performance are made.
TL;DR: In this article, a parametric investigation on the effects of temperature-dependent viscosity and viscous dissipation in simultaneously developing laminar flows of liquids in straight microchannels of constant cross sections is carried out.
Abstract: A parametric investigation is carried out on the effects of temperature-dependent viscosity and viscous dissipation in simultaneously developing laminar flows of liquids in straight microchannels of constant cross sections. Reference is made to fluid heating conditions with a uniform heat flux imposed on the walls of the microchannels. Six different cross sectional geometries are considered, chosen among those usually adopted for microchannels (circular, flat, square, rectangular, trapezoidal, and hexagonal). Viscosity is assumed to vary with temperature according to an exponential relation, while the other fluid properties are held constant. A finite-element procedure is employed for the solution of the parabolized momentum and energy equations. Due to the high value of the ratio between the length and the hydraulic diameter in microchannels, such an approach is very advantageous with respect to the one based on the steady-state solution of the elliptic form of the governing equations in a three-dimensio...
TL;DR: In this article, the performance of a U-shaped pulsating heat pipe (PHP) was investigated using numerical methods and the effect of the evaporator temperature on the pulse amplitude and frequency, rate of convection, and boiling heat transfer was also investigated.
Abstract: In this research the performance of a U-shaped pulsating heat pipe (PHP) was investigated using numerical methods. This heat pipe consists of two sections: The evaporator is set at the two ends of the pipe, and the middle part of the pipe comprises the condenser section. This heat pipe is a type of open looped pulsating heat pipe. The governing equations are derived analytically from the continuity, momentum, and energy equations and are solved implicitly. In this model, considering the liquid mesh, the rate of convection and boiling heat transfer in the U-shaped PHP, which has not been investigated as of yet, are examined. The effect of the evaporator temperature on the pulse amplitude and frequency, rate of convection, and boiling heat transfer is also investigated. The results show that by increasing the evaporator temperature, due to the increase in pulse amplitude and frequency, the rate of heat transfer due to convection and boiling in the pipe will increase too. Furthermore, it is derived that by i...
TL;DR: In this article, three different methods for two-phase flow modeling in microchannels and minichannels are presented, which are effective property models for homogeneous 2-phase flows, an asymptotic modeling approach for separated 2-phase flow, and bounds on twophase frictional pressure gradient.
Abstract: In this article, three different methods for two-phase flow modeling in microchannels and minichannels are presented. They are effective property models for homogeneous two-phase flows, an asymptotic modeling approach for separated two-phase flow, and bounds on two-phase frictional pressure gradient. In the first method, new definitions for two-phase viscosity are proposed using a one-dimensional transport analogy between thermal conductivity of porous media and viscosity in two-phase flow. These new definitions can be used to compute the two-phase frictional pressure gradient using the homogeneous modeling approach. In the second method, a simple semitheoretical method for calculating two-phase frictional pressure gradient using asymptotic analysis is presented. Two-phase frictional pressure gradient is expressed in terms of the asymptotic single-phase frictional pressure gradients for liquid and gas flowing alone. In the final method, simple rules are developed for obtaining rational bounds for two-phas...
TL;DR: In this article, the double decomposition method is used to analyze annular hyperbolic profile fin with variable thermal conductivity, and the results from the exact model solution are compared with the constant thermal conductivities case.
Abstract: In this article, the double decomposition method is used to analyze the annular hyperbolic profile fins with variable thermal conductivity. The double decomposition method is an advantageous way to solve the nonlinear problem. The solution gained by the double decomposition method is in the form of infinite power series, and the variable thermal conductivity is considered to have a linear relation with temperature. The results from the exact model solution are compared with the constant thermal conductivity case. The parameters that affect the fin performance and temperature distribution strongly are identified.
TL;DR: In this article, heat transfer and temperature behavior of complex flows encountered in the vehicle underhood compartment is experimentally studied and described with simple models, and models are proposed to predict the maximum temperatures and time constants of the underhood components as functions of the car speed and car engine power.
Abstract: In this work heat transfer and temperature behavior of complex flows encountered in the vehicle underhood compartment is experimentally studied and described with simple models. Underhood thermal measurements made on a passenger vehicle in a large-scale wind tunnel are reported here. The underhood is instrumented by 80 surface and air thermocouples and 20 fluxmeters. Measurements are carried out at three thermal functioning points, in all of which the engine is in operation and the front wheels are positioned on the test facility with power-absorption-controlled rollers. Models are proposed to predict the maximum temperatures and time constants of the underhood components as functions of the car speed and car engine power. The relative errors of the models are 3.6% and 3.7%, respectively. The maximum temperature and the time constant are crucial in the design and optimization of the underhood aerothermal management system. The results obtained in the present work also provide a large database for validati...
TL;DR: In this paper, a plate heat exchanger is used to create adsorbent layers only 4 mm thick between pairs of liquid flow channels of very low thermal mass, which has been evaluated under the European Union (EU) car air-conditioning testing conditions.
Abstract: A prototype compact sorption generator using an activated-carbon/ammonia pair based on a plate heat exchanger concept has been designed and built at Warwick University. The novel generator has low thermal mass and good heat transfer. The heat exchanger uses nickel brazed shims and spacers to create adsorbent layers only 4 mm thick between pairs of liquid flow channels of very low thermal mass. The prototype sorption generator manufactured has been evaluated under the European Union (EU) car air-conditioning testing conditions. While driven with waste heat from the engine coolant water (at 90°C), a pair of the current prototype generators (loaded with about 1 kg carbon in each of two beds) has produced an average cooling power of 1.6 kW with 2-kW peaks.
TL;DR: In this article, numerical study of combined natural convection and radiation heat loss from a modified cavity receiver of a solar dish collector using asymptotic computational fluid dynamics approach is presented.
Abstract: In this article, numerical study of combined natural convection and radiation heat loss from a modified cavity receiver of a solar dish collector using asymptotic computational fluid dynamics approach is presented. The natural convection and radiation heat losses are estimated for different angle of inclinations ranging from 0° (aperture facing sideways) to 90° (aperture facing downward). The numerical results are presented to show the effect of parameters such as Grashof number, angle of inclination, emissivity, temperature ratio, and diameter ratio on convection and radiation heat losses. Separate Nusselt number correlations for natural convection heat loss and combined convective and radiative heat loss are given using the method of asymptotic expansions. The heat loss model is comparable with well-known models. It is observed that the present heat loss model follows the same trend as that of the other heat loss models.
TL;DR: In this article, the effects of corona wind velocity, electrode spacing and air flow velocity on the level of evaporation enhancement were evaluated in two sets of experiments, i.e., with and without electric field, have been conducted.
Abstract: Enhancement of forced flow evaporation rate by applying electric field (corona wind) has been experimentally evaluated in this study. Corona wind produced by a fine wire electrode which was charged with positive high DC voltage impinges to water surface and leads to evaporation enhancement by disturbing the saturated air layer over the water surface. The study was focused on the effects of corona wind velocity, electrode spacing and air flow velocity on the level of evaporation enhancement. Two sets of experiments, i.e., with and without electric field, have been conducted. Data obtained from the first experiment were used as reference for evaluation of evaporation enhancement at the presence of electric field. Applied voltages ranged from corona threshold voltage to spark over voltage at 1 kV increments. The results showed that corona wind has great enhancement effect on the water evaporation rate, but its effectiveness gradually diminishes by increasing air flow velocity. Maximum enhancement ratios were...
TL;DR: In this article, the shape optimization of a rectangular channel with the opposite walls roughened by staggered arrays of dimples has been performed not only to enhance turbulent heat transfer but also to reduce friction loss.
Abstract: Shape optimization of a rectangular channel with the opposite walls roughened by staggered arrays of dimples has been performed not only to enhance turbulent heat transfer but also to reduce friction loss. The dimpled channel shape is defined by three geometric design variables, and the design points within design space are selected using Latin hypercube sampling. The shape of the channel is optimized with three-dimensional (3-D) Reynolds-averaged Navier–Stokes analysis and surrogate approximation methods. A weighted-sum method for multi-objective optimization is applied to integrate multiple objectives related to heat transfer and friction loss into a single objective. A weighted-average surrogate model is employed for this optimization. By the optimization, the objective function value is improved largely and heat transfer rate is increased much higher than pressure loss increase due to shape deformation. The optimum design results in lower channel height, wider dimple spacing, and deeper dimple. The fl...
TL;DR: In this paper, the effects of different roughness structures on internal flows in high-aspect-ratio rectangular microchannels were examined, and a model based on lubrication theory was compared to these results.
Abstract: This work aims to experimentally examine the effects of different roughness structures on internal flows in high-aspect-ratio rectangular microchannels. Additionally, a model based on lubrication theory is compared to these results. In total, four experiments were designed to test samples with different relative roughness and pitch placed on the opposite sides forming the long faces of a rectangular channel. The experiments were conducted to study (i) sawtooth roughness effects in laminar flow, (ii) uniform roughness effects in laminar flow, (iii) sawtooth roughness effects in turbulent flow, and (iv) varying-pitch sawtooth roughness effects in laminar flow. The Reynolds number was varied from 30 to 15,000 with degassed, deionized water as the working fluid. An estimate of the experimental uncertainty in the experimental data is 7.6% for friction factor and 2.7% for Reynolds number. Roughness structures varied from a lapped smooth surface with 0.2 μ m roughness height to sawtooth ridges of height 117 μ m....
TL;DR: In this paper, the adsorption isotherms of HFC-134a and activated carbon Maxsorb III measured using the constant-volume-variable-pressure method were presented.
Abstract: This article presents the adsorption isotherms of HFC-134a and activated carbon Maxsorb III measured using the constant-volume–variable-pressure method. The adsorption isotherms cover temperature ranges from 293 to 338 K and pressures up to 0.7 MPa. The trends of the experimental isotherms for activated carbon are found to be identical in all cases with previous studies except that the vapor uptake is slightly higher. The adsorption characteristic of the Dubinin–Ashtakov equation has been regressed from the experimental isotherms data and the maximum specific uptake is 2.15 kg of adsorbate adsorbed per kilogram of activated carbon. The heat of adsorption, which is concentration and temperature dependent, has also been extracted from the experiments.
TL;DR: In this paper, artificial neural network (ANN) approach has been utilized to characterize the thermohydraulic behavior of corrugated tubes combined with twisted tape inserts in a turbulent flow regime.
Abstract: In the research described here, artificial neural network (ANN) approach has been utilized to characterize the thermohydraulic behavior of corrugated tubes combined with twisted tape inserts in a turbulent flow regime. The experimental data sets were extracted from 57 tubes, 9 and 3 spirally corrugated tubes with varying geometries combined with 5 and 4 twisted tapes with different pitches. The tests were carried out with Reynolds numbers ranging from 3000 to 60,000. The experimental data sets have been utilized in training and validation of the ANN in order to predict the heat transfer coefficients and friction factors inside the corrugated tubes combined with twisted tape inserts, and the results were compared to the experimental data. The mean relative errors between the predicted results and experimental data were less than 2.9% for the heat transfer coefficients and less than 0.36% for the friction factor. The performance of the neural networks was found to be superior in comparison with the models c...
TL;DR: In this paper, a design optimization technique for mixing in a gas turbine combustor is presented, which entails the use of computational fluid dynamics and mathematical optimization to optimize the combustor exit temperature profile.
Abstract: In this article, a design optimization technique for mixing in a gas turbine combustor is presented. The technique entails the use of computational fluid dynamics and mathematical optimization to optimize the combustor exit temperature profile. Combustor geometric parameters were used as optimization design variables. This work does not intend to suggest that combustor exit temperature profile is the only performance parameter important for the design of gas turbine combustors. However, it is a key parameter of an optimized combustor that is related to the power output and durability of the turbine. The combustor in this study is an experimental liquid-fuelled atmospheric combustor with a turbulent diffusion flame. The computational fluid dynamics simulations use a standard k-ϵ model. The optimization is carried out with the Dynamic-Q algorithm, which is specifically designed to handle constrained problems where the objective and constraint functions are expensive to evaluate. The optimization leads to a ...