Gas Heat Transfer Research Articles

Physical and Numerical Modeling of Thermomechanical Processes in Gas–Air Systems of Piston Engines Under Gasdynamic-Nonstationarity Conditions

It is well known that as of today, internal combustion engines are the most widespread energy sources among heat engines. Therefore, one relevant problem in the development of world energy is to improve operating processes, and also to modernize systems and elements of piston internal combustion engines with the aim of improving their technical and economic indices. In the present paper, the authors have given new information on nonstationary gasdynamics and local heat transfer of pulsating flows in gas–air flow ducts of internal combustion engines, and also have proposed methods for improving the processes in intake and exhaust systems. Experimental investigations were conducted on full-scale models of a single-cylinder internal combustion engine with supercharging and without it. Physical features of pulsations of gas flows in the engines′ gas–air flow ducts have been described. Calculated and experimental dependences of the change in the instantaneous velocity and the pressure of the gas flow in the gas–air flow ducts with time have been presented. Particular emphasis was placed on an analysis of the intensity of heat transfer in gas–air flow ducts of different configurations. It has been shown that lateral profiling of intake and exhaust pipelines exerts a positive influence on the technical and economic indices of piston engines without supercharging. A method to reduce pulsations of the pressure and velocity of gas flows (on the average, by a factor of 2) in the intake pipeline of a supercharged internal combustion engine has been proposed, which leads to an improvement of the reliability of the entire engine.

Semiempirical Model of Convective Heat Transfer of Turbulent Gases

On the basis of the Prandtl semiempirical wall-turbulence hypothesis, the author has substantiated theoretically the possibility of setting boundary conditions to the equations of a mathematical model of turbulent motion and convective heat transfer of a gaseous medium on a coarse grid. It has been shown that the results of numerical simulation are in agreement with experimental data on the heat transfer of air and high-temperature gases in tubes.

Modeling of Heat and Mass Transfer in Heat-Shielding Composite Materials Based on the Universal Law of Binder Decomposition

A physical and mathematical model of heat and mass transfer under phase transformations during the high-temperature, aerodynamic heating of high-speed aircraft is constructed in this work based on the identified law of the decomposition of binders of most heat-shielding composite materials. The mathematical model includes a description of the occurrence and advancement of the binder-decomposition zone (pyrolysis) limited by two moving boundaries of phase transformations, the heat transfer and filtration of pyrolysis gases in the porous coke residue, their injection into the gasdynamic boundary layer, and the distribution of the temperature and density of the composite material in the zone pyrolysis. The mathematical model and analytical method for the solution of the nonlinear heat and mass transfer problem allow the Stefan-type problem to be reduced to the solution of the transcendental equation with respect to the mass velocity of the pyrolysis zone. The results were obtained for temperature fields with allowance for the filtration in the porous residue and the pressure distribution of pyrolysis gases and for temperature fields in the pyrolysis zone that were not affected by binder decomposition, as well as the distribution of pyrolysis gas density in the pyrolysis zone.

Optimising graphite composites and plate heat exchangers for latent thermal energy storage using measurements and simulation

Low thermal conductivity is a significant issue for the deployment of phase change based thermal energy stores. A structured methodology is required to find the best overall design, in order to assess its techno-economic feasibility. In this study the optimised structural configuration of graphite composites for this purpose are first considered from three different perspectives using both experimental measurements and numerical simulations. Both steady state and dynamic performance are taken into account, with the latter being considered both with and without phase change. In all three cases it is demonstrated that thin, parallel graphite sheets offer the most optimal results compared to other options. Resulting in the highest composite thermal conductivity measured to date at a loading of 3.5 mass%. The next step in achieving the best overall performance is the optimisation of this composite in the context of a plate and frame heat exchanger. For this purpose a simplified analytical model of this configuration was developed and validated. This was used in conjunction with the log mean temperature difference approach to optimise the exchanger for two typical applications: low temperature, low duty and high temperature, high duty. The results indicate that, even under an idealised configuration, the system will only be suitable for high temperature applications with gas heat transfer fluids and short discharge times (~1 h). For low temperature applications it may be possible to use liquid heat transfer fluids but also only for short discharge times. From an economic perspective, the range of graphite sheets currently available on the market will have to be reduced in cost by two or more orders of magnitude in order to be cost-effective.

An elaborated diffusion mathematic model of radiative transfer in an extinction medium

The computational method of diffusion approach, used sometimes for simulating the radiative heat transfer, differs in simplicity of its algorithm, but can not take ac-count of radiation anisotropy. The reasons of its insufficient accuracy have been re-vealed; it is shown that inadequate boundary con-ditions produce unpredictable er-rors. Ways for eliminating short-comings of the diffusion method have been developed. The accurate diffusion model has been theoretically elaborated for radiative heat transfer in dust-laden gases. To eliminate singularity of temperature curves on the surface of confining walls, the en-closed extinction medium was hypothetically considered as an unbounded one. The radiation intensity has been expanded in a se-ries and integrated over the spherical solid angle, what has allowed to obtain more precise differential equation for the resulting radiation flux both in the unbounded and enclosed extinction medium. As a result, shortcomings peculiar to the previous method of diffusion approach have been eliminated, and correct Neumann and Robin boundary conditions have been formulated. As a result, the elaborated diffu-sion model got the increased accuracy in simulating radiative heat transfer, and con-currently it offers an advantage of simplicity and universality of the diffusion ap-proach method.

Based on the analysis and discussion of heat conduction of machining in power system

Heat transfer process in power system applications mainly refers to the gas in the cylinder through the combustion chamber wall and related parts pass the process of cooling medium, the heat it mainly includes the gas flow and combustion in cylinder, cylinder internal boundary layer heat transfer (gas and wall heat transfer), heated component of thermal load and thermal conductivity, cooling medium heat transfer, and a series of process. Because it is closely related to the working process of the power system, the lubrication cooling system and the structural strength design of components, the heat transfer and its thermal load will directly affect the dynamic performance, economy and reliability of the system. Therefore, the heat conduction of machining in the power system has always been an important field in the power system research.

Effect of Slurry Coating on the Resistance of Thermal Insulation Insert in Blast Furnace Air Tuyere

Air tuyeres account for 30% of all heat losses in a blast furnace. One of the methods for lining a tuyere from the air passage side is to use a thermal insulation insert. However, certain defects of the insert on the working surface of the air passage along with thermal stresses generated therein may lead to its premature failure during operation. This paper discusses applying a slurry coating to the inner surface of the insert to prevent its premature cracking and improve performance. The effect of a slurry-coated thermal insulation insert on gas dynamics and heat transfer inside a blast furnace air tuyere was studied, and simulation of the above processes was performed. It was shown that applying a slurry coating to the lower portion of the thermal insulation insert is most beneficial for increasing the heat flux and raising the air-blast temperature at the tuyere outlet, while applying the coating to the entire inner surface of the insert helps improving its resistance, which has been confirmed experimentally.

A finite-volume numerical model for temporal and spatial variability of methane oxidation in landfill covers

Abstract A multi-field model is proposed to study methane oxidation in landfill covers, especially its temporal and spatial variability. The proposed model consists of water-gas two-phase flow, multi-component gas flow, heat transfer, and biochemical reaction modules, and is solved using OpenFOAM based on the finite volume method. Applying this model, the coupled effect of gas diffusion, temperature and oxidation rate is studied, and the influence of water content on oxidation efficiency and capacity is quantified. The temporal variability of oxidation efficiency and capacity in landfill covers is illustrated by modelling a long-term field test. It is found that the CH4 removal rate can reach 40 g d−1 m−2 in the summers and drop to less than 10 g d−1 m−2 in the winters. A 3-D landfill cover cell with a cracked zone is applied to illustrate the spatial variability of oxidation, and a significant preferential flow can be observed when the cracked zone depth of a 1 m thick soil cover exceeds 0.8 m. The above studies demonstrate that the proposed model is able to predict temporal and spatial distributions of methane oxidation efficiency and capacity, and some results contribute to a better understanding of methane oxidation in landfill covers.

Simulation-based optimisation of thermodynamic conditions in the ESEM for dynamical in-situ study of spherical polyelectrolyte complex particles in their native state

We present a complex analysis and optimisation of dynamic conditions in the environmental scanning electron microscope (ESEM) to allow in-situ observation of extremely delicate wet bio-polymeric spherical particles in their native state. According to the results of gas flow and heat transfer simulations, we were able to develop an improved procedure leading to thermodynamic equilibrium between the sample and chamber environment. To quantify and hence minimise the extent of any sample deformation during specimen chamber pumping, a strength-stress analysis is used. Monte Carlo simulations of beam-gas, -water, and -sample interactions describe beam scattering, absorbed energy, interaction volume and the emission of signal electrons in the ESEM. Finally, we discuss sample damage as a result of drying and the production of beam-induced free radicals. Based on all experimental and simulation results we introduce a Delicate Sample Observation Strategy for the ESEM. We show how this strategy can be applied to the characterization of polyelectrolyte complex spherical particles containing immobilized recombinant cells E. coli overexpressing cyclohexanone monooxygenase, used as a model biocatalyst. We present the first native-state electron microscopy images of the viscous core of a halved polyelectrolyte complex capsule containing living cells.

Моделирование и исследование динамики адсорбционного разделения синтез-газа и концентрирования водорода

On the basis of the Dubinin theory of micropore volume filling, a mathematical model of dynamics of pressure swing adsorption processes for synthesis gas separation has been developed. The model takes into consideration the influence of the processes of mass and heat transfer in gas and solid phases on the kinetics of diffusion transfer of adsorbate (carbon dioxide, carbon monoxide, hydrogen) in the adsorbent layer and accounts for all devices included in the process diagram (adsorber, compressor, vacuum pump, valves, throttle, receiver). Numerical studies of the process of separation of synthesis gas and concentration of hydrogen in a four-adsorber unit with granulated zeolite adsorbent 13X were carried out by methods of mathematical modeling: the influence of disturbing influences (composition and temperature of the initial hydrogen-containing gas mixture), regime parameters (cycle duration, pressure at the compressor outlet, pressure at the vacuum pump inlet, backflow coefficient) and design parameters (length of the adsorbent bulk layer and inner diameter of the adsorber) on the purity of the product hydrogen, its recovery rate and productivity of the unit were studied. The most dangerous disturbances and the most effective regime parameters of pressure swing adsorption process of synthesis gas separation were determined. It is established that the increase of temperature from 298 to 323 K and decrease of hydrogen concentration from 68 to 48 % (vol.) in initial gas mixture result in ~10 % lower efficiency of the unit due to the decrease of product hydrogen recovery rate. Practical recommendations on effective choice of operation regimes of an adsorption unit to ensure the achievement of required purity of product hydrogen at the level of 99.99 % (vol.), regardless of the impact of disturbances are formulated.

Numerical analysis of the influence of side wall shape on the efficiency of thermal energy storages based on granular phase change materials

This work is devoted to a numerical study of processes in thermal energy storages based on granular phase change materials. The influence of the side walls shape on the efficiency of such heat accumulators is studied when the plane-parallel flows of gas heat transfer fluid take place. The shape of the energy storage affects the heat transfer fluid flow in the object, and this affects the heat transfer, heat accumulation and heat recovery. Using the novel numerical model, the influence of narrowing and expansion of the side walls on the charging and discharging processes of thermal energy storages with rectangular cross sections are studied under two types of boundary conditions: the known mass flow rate of gas at the object inlet and the known gas pressure drop at its open borders for different phase change temperatures of the phase change material .Different efficiency criteria are used to estimate the preferred shape of a heat storage. For the charging process, the preference criteria considered are the maximum instantaneous storage efficiency, the maximum cumulative storage efficiency, and the minimum time to fully charge the device. For the discharge process, the considered preference criteria are the maximum energy recovery efficiency, the maximum total utilization ratio, and the maximum time to maintain the temperature of the heat transfer fluid at the outlet not lower than the desired value. It is shown that the preferred shape of the energy storage depends on the choice of the efficiency criterion and specific process conditions such as boundary conditions, phase transition temperature, etc. Narrowing and expanding thermal energy storages have an advantage in rare cases, and storages with straight walls are often most preferable.

Numerical Investigation on Heat and Mass Transfer of Volatile Flammable Gases Within the Nuclear Reactor Containment

Abstract Diffusion of volatile flammable species in the air can cause a fire risk within the nuclear reactor containment. However, computational prediction on species concentration distributions remains significantly difficult due to a shortage of multicomponent diffusion coefficients. In this work, considerable effort has been made to calculate concentration distributions of formaldehyde and benzene vapor volatilized from radiation-proof coatings of reactor containment walls. For this purpose, a numerical model is proposed to simulate species transport and concentration distributions due to full multicomponent diffusion and thermal diffusion. Meanwhile, the in-house UDFs' source code is programmed for solving diffusivities and essential thermophysical properties. After compiling and linking the source code with the numerical model, a pressure-based SIMPLE algorithm is imposed for pressure–velocity coupling calculations. Computational results indicate that concentration distributions are highly dependent on the fluid motion as well as potentially flammable areas decrease gradually with increased ventilation rates. Also, primary and secondary vortices are symmetrically distributed about the vertical centerline of the reactor containment as well as triangular secondary vortices can significantly suppress concentrations of formaldehyde and benzene vapor at the bottom portion of the containment. Finally, excellent agreement is observed between computational results and analytical solutions.

Sequences of Sub-Microsecond Laser Pulses for Material Processing: Modeling of Coupled Gas Dynamics and Heat Transfer

Multipulse laser processing of materials is promising because of the additional possibilities to control the thickness of the treated and the heat-affected zones and the energy efficiency. To study the physics of mutual interaction of pulses at high repetition rate, a model is proposed where heat transfer in the target and gas-dynamics of vapor and ambient gas are coupled by the gas-dynamic boundary conditions of evaporation/condensation. Numerical calculations are accomplished for a substrate of an austenitic steel subjected to a 300 ns single pulse of CO2 laser and a sequence of the similar pulses with lower intensity and 10 μs inter-pulse separation assuring approximately the same thermal impact on the target. It is revealed that the pulses of the sequence interact due to heat accumulation in the target but they cannot interact through the gas phase. Evaporation is considerably more intensive at the single-pulse processing. The vapor is slightly ionized and absorbs the infrared laser radiation by inverse bremsstrahlung. The estimated absorption coefficient and the optical thickness of the vapor domain are considerably greater for the single-pulse regime. The absorption initiates optical breakdown and the ignition of plasma shielding the target from laser radiation. The multipulse laser processing can be applied to avoid plasma ignition.

Fully Coupled Multi-Scale Model for Gas Extraction from Coal Seam Stimulated by Directional Hydraulic Fracturing

Although numerous studies have tried to explain the mechanism of directional hydraulic fracturing in a coal seam, few of them have been conducted on gas migration stimulated by directional hydraulic fracturing during coal mine methane extraction. In this study, a fully coupled multi-scale model to stimulate gas extraction from a coal seam stimulated by directional hydraulic fracturing was developed and calculated by a finite element approach. The model considers gas flow and heat transfer within the hydraulic fractures, the coal matrix, and cleat system, and it accounts for coal deformation. The model was verified using gas amount data from the NO.8 coal seam at Fengchun mine, Chongqing, Southwest China. Model simulation results show that slots and hydraulic fracture can expand the area of gas pressure drop and decrease the time needed to complete the extraction. The evolution of hydraulic fracture apertures and permeability in coal seams is greatly influenced by the effective stress and coal matrix deformation. A series of sensitivity analyses were performed to investigate the impacts of key factors on gas extraction time of completion. The study shows that hydraulic fracture aperture and the cleat permeability of coal seams play crucial roles in gas extraction from a coal seam stimulated by directional hydraulic fracturing. In addition, the reasonable arrangement of directional boreholes could improve the gas extraction efficiency. A large coal seam dip angle and high temperature help to enhance coal mine methane extraction from the coal seam.

Study on Dynamic Heat Transfer Process of Waste Gas in Coke Oven Riser Coking Cycle

Waste heat recovery of waste gas in rising tube is a complex heat transfer process, and this paper analyses and studies the heat transfer problem of the coke hoist heat exchanger in 1 coking cycle by theoretical derivation and numerical calculation. Based on the thermal balance equation, the heat transfer differential equation of the gas flow of the rising tube is derived, the mathematical model of heat transfer for waste gas waste heat recovery is established, and the heat transfer and heat transfer coefficient of the coke oven rising tube heat exchanger in 1 coking cycle are obtained. In 1 coking cycle, the heat transfer of waste gas fluctuates in 1 coking cycle, and the heat transfer of waste gas in the rising tube is 19 to 26 kW, with an average heat transfer coefficient of 30 to 36 W/(m2·K). In the late coking, with the decrease of waste gas flow and the decrease of heat transfer, it will cause the temperature of the rising tube wall to change, and the gas flow should be maintained in the late stage of coking to prevent the phenomenon of sudden change of the inner wall temperature; In the actual production can be several coke ovens in parallel production, and the flow and temperature of waste gas timely monitoring, so that the flow of waste gas control at 800m3/h, improve the efficiency of waste gas waste heat recovery.

Gas Flow and Heat Transfer in an Enclosure Induced by a Sinusoidal Temperature

The heat transfer and flow characteristics of rarefied gas confined within a square cavity are investigated. The cavity upper-wall is subjected to a symmetrical sinusoidal temperature with respect to its midline. Two cases are considered, one of them is that the bottom and two sidewalls are kept adiabatic, while in the other, all the enclosure walls are considered diffusely reflecting. Kinetically, the gas is simulated with the direct simulation Monte Carlo method (DSMC) in the slip and transition regimes. The DSMC results are compared with the Navier-Stokes-Fourier equation, with second order boundary conditions of velocity slip and temperature jump, (NSF) in the slip regime. Main outcomes are presented as velocity streamlines overlaid on temperature contours and macroscopic parameters plots. The Knudsen number (Kn) increase shows other vortices, beside the two classical ones that appear in the hydrodynamic and slip regime. The normal heat flux about the sinusoidally heated wall is strengthened by the flow in DSMC rather than that predicted by NSF method. Good agreement is observed between the NSF theory and DSMC method in the early slip regime, but when Kn=0.1 the NSF approach breaks down to show the secondary counter rotating eddies illustrated by the DSMC method.

Automated Decision Support System in the Energy- and Resource-Efficiency Management of a Chemical-Energy Engineering System for Roasting Phosphorite Pellets

Mathematical and computer-based models of a complex chemical-engineering roasting process as an interdependent plurality of three processes—drying, calcining, and sintering of moving, tight, and multilayered mass of phosphorite pellets in conveyer roasting machine, differing in attention to the intensity of processes of internal moisture transfer within the pellet and overwetting processes of separate horizons in heated pellet layer, have been developed, making it possible to the determine engineering parameters of the roasting mode. The adequacy of the mathematical model is checked from the comparative analysis data of calculated values of the moisture content and the temperature of pellets and parameters of heat-transfer gas, as well as moisture-transfer intensity in pellets when they are dried in the moving tight layer, were compared with the data of industrial testing. Numerous calculated experiments to determine the relative extent of drying, the moisture content, the intensity of pellet drying, and the moisture content of heat-transfer gas are carried out, in which crude pellets and the engineering data of the operating mode of a roasting machine have different parameters. An informal and mathematical statement of the problem of optimizing the chemical-energy engineering process (CEEP) for roasting the moving tight multilayered mass of phosphorite pellets within the complex chemical-energy engineering system (CEES) of the conveyer roasting machine is developed as the problem of discrete dynamic programming in light of spatiotemporal multistage processes for roasting the moving multilayered pellet mass, the intensity of the internal moisture transfer processes within the pellet, processes of overwetting the separate layers of pellets, and variables of the control stream of the heat-transfer gas; this makes it possible to enhance energy efficiency by means of intensifying heat-and-mass transfer processes of multilayered drying, calcining, and sintering. The performance criterion is the minimum cost of electrical and heat energy expended on the CEEP for roasting. The results were used to calculate energy efficient pellet roasting in CEES of the conveyer roasting machine. It has been found that, under the optimum mode of multilayered pellet roasting, there is no overwetting zone, processes of heat and moisture transfer are intensified, energy consumption decreases, and the quality of the end product increases. The problem of pellet overwetting in separate layer horizons of the drying zone of the roasting machine has been studied. The actual theoretical and practical problem of energy and resource saving in roasting pelletized crude ore in the tight layer has been solved. The mathematical model of heat-mass exchange in the pellet layer and testing its adequacy have been presented. The problem of optimizing power inputs based on the intensification of roasting processes has been solved.

Numerical analysis of flow characteristics and heat transfer of high-temperature exhaust gas through porous fins

Engine waste heat recovery technology based on thermodynamic cycles calls for highly compact and effective exhaust-gas heat exchangers, especially in the space-constrained applications. A porous fins heat exchanger was proposed and compared with the traditional solid fins heat exchanger in order to effectively enhance the heat transfer and realize the lightweight design. A three-dimensional model of the porous fins exhaust gas heat exchanger under forced convection has been developed and numerically investigated based on the Darcy-Forchheimer-Brinkman momentum equation and local thermal non-equilibrium energy equation. Computational studies are carried out to compare the hydraulic and thermal performance between the porous fins and solid fins of equal size at Reynolds number ranging from 5000 to 50,000. Results show that the porous fins heat exchanger greatly improves by 73.2–77.1% while brings an increase of 15.1–18.5% in the pressure drop. In terms of overall performance, the weight of porous fins heat exchanger is reduced by 90%, and the comprehensive performance factor measuring heat transfer and pressure drop is increased by 41% compared with solid fins heat exchanger. Subsequently, the effects of porous fins height, porous fins density, the porosity and pore density on the heat exchanger performances are also discussed. It is found that there is an optimal fin height to achieve the best comprehensive performance of porous fins heat exchanger.

Heat and mass transfer during a sudden loss of vacuum in a liquid helium cooled tube – Part II: Theoretical modeling

A sudden loss of vacuum in particle accelerator beamlines and other cryogenic systems can lead to substantial equipment damage and possible personnel injuries. Developing a clear understanding of the complex dynamical heat and mass transfer processes involved following a sudden vacuum break is of great importance for the safe operation of these systems. Our past experimental studies on sudden vacuum break in a liquid helium cooled tube revealed a nearly exponential slowing down of the propagating gas front. However, the underlying mechanism of this slowing down is not fully explained. In this paper, we discuss a theoretical framework that systematically describes the gas dynamics, heat transfer, and mass deposition of the propagating and condensing gas inside the helium-cooled tube. The experimentally observed apparent gas-front propagation, measured as the abrupt temperature rise by the thermometers installed along the tube wall, can be well reproduced by the model simulation. We also show that following the gas front, the mass deposition rate of the gas on the tube inner wall approaches a constant. The extension of this nearly constant gas deposition zone is the key to understand the observed exponential slowing of the gas propagation. Our model also allows us to gain valuable insights about the growth of the frost layer on the tube inner surface. This work paves the way for a theoretical understanding of the physical processes involved during vacuum break in accelerator beamlines.

Numerical study of slug flow heat transfer in microchannels

In the present paper, the heat transfer of slug flow in rectangular microchannels is studied numerically. The Al2O3-water nanofluid with the volume fraction of 1% is used as a homogeneous liquid. Three different configurations are considered to investigate the hydrodynamics and heat transfer in the microchannels. Boundary mesh adaptation method is used to capture the liquid film around gas bubbles. Slug flow characteristics are simulated with a very low computational time compared to previous researches. Slug and bubble lengths, liquid film properties, pressure drop, gas void fraction and heat transfer characteristics are studied. Results show that different inlet configurations impact on slug and bubble lengths. Gas hold-up is influenced by vortices and high frequency fluctuations that are created in the liquid film due to the difference between gas and slug acceleration. It is found that the Nusselt number is an increasing function of slug length and unit cell length. The results reveal that the hottest point on the wall is where the back tail vortex is generated. The nanofluid enhances the heat transfer coefficient by 10% in comparison with the pure water.