Gas Heat Transfer Research Articles

Influence of the wall temperatures of the combustion chamber and intake ports on the charge temperature and knock characteristics in a spark-ignited engine

Reducing wall temperatures is a promising method to suppress knocking behavior in spark-ignited engine. However, this may increase undesirable heat loss which acts as countereffect, so a strategic cooling approach is required. In this study, a multidisciplinary investigation of the wall temperature effect was demonstrated using experiments and simulations. By experiments under full load and part load conditions, improvements in the indicated thermal efficiency achieved by knock-limited spark advancement were obtained, and detailed analyses were incorporated. Under cooled conditions, it was found that an improved thermal efficiency was achieved by not only the advanced combustion phasing but also the reduced compression work obtained from increased gas density, particularly under part load conditions. By categorizing and evaluating the heat transfer phases using simulations, it was found that the cooled wall temperature did not provide a significant gas temperature drop via compression and combustion processes. Unexpectedly, a notable contribution to gas heat transfer reduction arose during the early gas induction stage because of not only the extended period of heat transfer but also the large surface area and initial low temperature before compression. An enhanced cooling on cylinder head resulted in a larger effect on knock mitigation than enhanced liner cooling under normal conditions, attributed to the large heat transfer at the intake port wall. From the assessment, as the liner coolant dominated the piston surface, it was found that the contribution of the liner wall temperature to the gas temperature reduction was significantly influential, even showing a higher knock mitigation effect after intake port insulation was applied. Intensified tumble flow showed a high potential of gas temperature decrease by increasing the heat transfer from gas to wall during the compression stroke, and the effect of the enhanced cooling on the liner was more significant than that of the normal intake port due to the high velocity and turbulence of the air. The simulation results revealed that enhanced liner cooling could decrease the in-cylinder temperature by more than 18 K when insulation and intensification were both applied to the intake port design.

Investigation of hydrodynamics and convection in the porous car heat exchanger

Car engine cooling is actual problem today. The work considers the gas flow in aluminium open cell foam material with two different values of the porosity of the medium 0.9 and 0.7. Models of porous media with an ordered and random structure have been created; in both cases, the porous medium is constructed by a set of intersecting spheres. Numerical modelling of gas flow and heat transfer in a porous medium is carried out using the control volume method in the ANSYS Fluent software package (v. 19.0). The results of the calculation of the pressure drop correlate well with the results of calculations obtained by the semi-empirical dependence. Given the best mixing of the flow, convective heat transfer is more intense in the model of a porous medium with a random arrangement of cells. The highest porosity of medium 0.9 provides the lowest pressure drop and increases the value of heat transfer. It has been shown that in a porous medium of any geometry, the Nusselt number increases with increasing Reynolds number. In this case, open cell foam with a random arrangement of pores in space and a maximum porosity of the medium is preferred for use as a heat exchanger in transport.

Numerical analysis of swirling turbulent droplet-laden flow and heat transfer in a sudden pipe expansion

The effect of swirling intensity on the structure and heat transfer of a turbulent gas–droplet flow after a sudden pipe expansion has been numerically simulated. Air is used as the carrier phase, and water, ethanol, and acetone are used as the dispersed phase. The Eulerian approach is applied to simulate the dynamics and heat transfer in the dispersed phase. The gas phase is described by a system of Reynolds-averaged Navier-Stokes (RANS) equations, taking into account the effect of droplets on mean transport and turbulent characteristics in the carrier phase. Gas phase turbulence is predicted using the second-moment closure. A swirling droplet-laden flow is characterized by an increase in the number of small particles on the pipe axis due to their accumulation in the zone of flow recirculation and the action of the turbulent migration (turbophoresis) force. A rapid dispersion of fine droplets over the pipe cross-section is observed without swirling. With an increase in swirling intensity, a significant reduction in the length of the separation region occurs. The swirling of a two-phase flow with liquid droplets leads to an increase in the level of turbulence for all three types of liquid droplets investigated in this work due to their intensive evaporation. It is shown that the addition of droplets leads to a significant increase in heat transfer in comparison with a single-phase swirling flow. The greatest effect of flow swirling on heat transfer intensification in a two-phase gas-droplet flow is obtained for the droplets of ethanol and water and smallest effect is for the acetone droplets.

RETRACTED: A new fractal model on fluid flow/heat/mass transport in complex porous structures

Abstract A new fractal theoretical model with periodic pore morphology, which idealizes the pore channels of the porous media as gourd-shaped structure, is established to model the transport in complex porous media. Based on the fractal model, the flow/thermal/mass transport properties of single-phase fluid and two-phase fluid flowing through porous structures are predicted, and analytical solutions to the permeability, thermal conductivity, diffusion coefficient of single-phase fluid in porous media, and the permeability, and the thermal conductivity of two-phase fluid in unsaturated porous media are derived. The analytical results of the proposed model for predicting the single-phase gaseous flow, heat and mass transfer in porous media are compared with the previous model data based on the idealized tubule-shaped structure of the real pore channels in porous media, which shows that the previous tubule-shaped structure is a limiting case of the present gourd-shaped structure. Analytical expressions of absolute permeability, relative permeability, and effective thermal conductivity of two-phase flow and heat transport in unsaturated porous media are derived for the present model and the previous tubule model. A parametric analysis is performed for presenting the effects of the structural parameters and fluid properties on the effective thermal conductivity, permeability and effective diffusion coefficient predicted. Meanwhile, the results show that the predicted data of the theoretical model are in good agreement with experimental data. The present study is significant for enhancing the flow, heat and mass transfer in accurately predicting transport properties of real porous media applications, and can provide an important parameter basis and guidance for the design of the transport system incorporating porous media.

Effect of side walls shape on charging and discharging performance of thermal energy storages based on granular phase change materials

Abstract In this work, thermal energy storages based on granular phase change material with gaseous heat transfer fluid are studied to determine their optimal shape. The shape of the energy storage unit affects the gas dynamics in the object, and this affects the processes of heat transfer, heat accumulation and heat recovery. A novel numerical model is developed and used for investigating the gas-dynamic and thermophysical processes in packed bed thermal energy storages. Using numerical experiments, the effect of narrowing and expansion of side walls on charging and discharging processes of the energy storages is studied for two types of boundary conditions: the known input mass flow rate of the gas and the known gas pressure at the open boundaries of the object. To determine the most optimal shape of the thermal energy storage, several efficiency criteria are used. It is shown that preference of the storage shape at both charging and discharging processes depends on the choice of efficiency criterion and specific conditions of the process such as boundary conditions, phase change temperature of granular material, and so on. Narrowing or expanding thermal energy storages have advantages in rare cases; the storages with straight walls are often most preferable.

Electronic and Simple Oscillatory Conduction in Ferrite Gas Sensors: Gas-Sensing Mechanisms, Long-Term Gas Monitoring, Heat Transfer, and Other Anomalies.

The early detection and warning of the presence of hazardous gases have been well studied. We present a study that focuses on some fundamental properties of gas sensors for liquefied petroleum gas (LPG) using spinel nanoferrites, namely, CoSm0.1Fe1.9O4, CoCe0.1Fe1.9O4, MgCe0.1Fe1.9O4, and MgFe2O4. A highly sensitive and selective response of 846.34 at 225 °C toward 10,000 ppm concentration of LPG was recorded. Other flammable gases tested were hydrogen, methane, propane, and butane. Electronic conduction of LPG sensors near saturation showed simple electrical oscillations that can be attributed to the self-dissociation of water molecules physically adsorbed on the surface of the chemisorbed oxygen species due to proton transfer. The oscillatory behaviors follow fluctuations in the operating temperature attributed to heat transfer between the physisorbed water molecules and the hot sensor surface. This depends on the LPG concentration because higher LPG concentration gives rise to greater heat transfer from the sensors. The adsorption and desorption of these water molecule multilayers take a few hundreds of seconds at low concentrations, while the adsorption formation process takes longer at higher concentrations. Other parameters such as LPG exposure time, bias voltage, relative humidity, ambient conditions, operating temperatures, and temperature of the gas not only affect electrical oscillations and thermal fluctuations but also switch the dominant charge carriers from p- to n-type or vice versa. The type of sensor surface, either p- or n-type, did not appear to affect the oscillatory behavior, while the exposure time, short or long, determined the appearance and further behavior of the oscillations. The long-time exposure to 10,000 ppm concentration resulted in the resistance gradually decreasing due to the lack of oxygen supply, while at 5000 ppm, this was constant, stable, and oscillated indefinitely. Changing the dry air to argon gas as a carrier and for dilution of the hazardous gas prevented the electrical oscillations and thermal fluctuations and significantly lowered the response values. Both the inert ambient (argon gas) and changing operating temperature flipped the dominant charge carriers of these sensors. The concentration of these chemisorbed oxygen species governs the charge space and depletion layers. In addition, the spinel nanoferrites used contained higher oxygen vacancies than the lattice oxygen and chemisorbed oxygen. When using dry air, the oscillations were observed at 3000 ppm concentration, while using argon gas, they were observed at 7000 ppm concentration. The room-temperature LPG responses were about 35 and 80 under 45% relative humidity using dry air and argon gas, respectively. These room-temperature measurements showed electrical oscillations but did not show any thermal fluctuations or heat transfer phenomena. This study presents a deeper insight into the fundamentals of gas-sensing mechanisms and energy costs involved.

Hydrogen storage using liquid organic carriers: Equilibrium simulation and dehydrogenation reactor design

Liquid organic hydrogen carriers (LOHC) are unsaturated organic compounds used for chemical hydrogen storage. Using an equilibrium model of the LOHC N-ethylcarbazole, we discuss potential efficiency increases of hydrogen storage systems based on N-ethylcarbazole by the integration of low-temperature waste heat. N-ethylcarbazole is well suited for pressure swing operation with heat exchange between hydrogenation and dehydrogenation. We present and discuss kinetic data of the dehydrogenation reaction gathered in a tubular reactor that was mounted in different orientations and flow configurations. Similar maximum values of power density are reached in vertical and in horizontal orientation. Vertical orientation allows the favorable operation with counter-flow of the liquid carrier and the evolved hydrogen gas and radial heat transfer is significantly better than in horizontal orientation. In vertical reactor configurations, catalyst efficiency and operational stability are impaired at high void fractions. This issue can be reduced by dehydrogenation at elevated pressure and intermediate gas separation from the catalyst bed.

A Correlation for Nusselt Number of Slip Gas Flow in Confined Porous Media

Abstract A clear understanding of flow and heat transfer at pore-scale level in microporous media is a topic of concern in microcooling/heating systems. In this work, a multiple-relaxation-time lattice Boltzmann method (LBM) is employed to study flow and heat transfer of gas in microporous media. Curved boundaries are treated using an effective boundary condition, which is formed by combining nonequilibrium extrapolation with counterextrapolation methods. The method also incorporates velocity slip and temperature jump on gas–solid interface. A two-dimensional (2D) porous domain composed of microcylinders, is considered from a representative element volume (REV) for the simulation. Porosity of the domain is variated by altering diameter of microcylinders. Nusselt number is calculated by varying Knudsen number (0.0–0.1), Reynolds number (5–50) and porosity (0.4–0.8). Based on the obtained numerical predictions, a new Nusselt number correlation is proposed for the first time in this work which can accurately predict the heat transfer for slip gas flow in confined porous media.

Thermal response of high-aspect-ratio hydrogen cylinders undergoing fast-filling

Fast-filling can lead to excessive heating of a hydrogen cylinder. Gas cylinders with large length-to-diameter aspect ratios are prone to developing hot-spots that may cause the temperature of the structure locally to exceed specified limits. The hot-spots develop because convective heat transfer into the cylinder wall is not uniform along the length of the cylinder. Computational fluid dynamic simulations of fast-filling reveal that the flow within large aspect ratio cylinders is characterised by two distinct flow patterns. The region near to the inflow nozzle exhibits an axi-symmetric recirculating flow pattern that extends to approximately three cylinder diameters downstream from the inlet. Wall heat transfer in the recirculation region of the flow is driven strongly by the jet of turbulent fluid from the nozzle. The wall heat transfer in the recirculation region rapidly develops quasi-steady behaviour in which the Nusselt number is approximately proportional to the inlet jet Reynolds number. Downstream of the recirculation zone there is a region of axial flow, in which wall heat transfer occurs at a slower rate, driven by decaying turbulence transported from the recirculation zone. The reduced heat transfer in the end gas leads to a higher gas temperature but also to a lower structural temperature downstream of the recirculation zone. Analysis shows that wall heat transfer can be enhanced and peak gas temperatures reduced by dividing the hydrogen injection between separate nozzles. Directing the nozzles axially, ideally at intervals of around 4 cylinder diameters, increases heat transfer and prevents the formation of hot regions of stagnant fluid. This approach allows more hydrogen to be stored for a given pressure or gas temperature limit. However the analysis across various length-to-diameter ratios and nozzle configurations reveals substantially different trends for the gas temperatures appearing in ISO safety specifications, and the temperature of the structural materials they seek to safeguard.

Fabrication and Characterization of Porous Silica/Carbon Nanotube Composite Insulation

AbstractIn recent years, the demand for high performance thermal insulations has increased. While foam and aerogels have been researched for high performance thermal insulation, novel material design is required for further improvement. A porous silica has been found to have the potential to form a new thermal insulation material. However, porous silica is a powder and is difficult to form the porous compact. Therefore, we propose a composite insulation of powdered porous silica (p-SiO2), carbon nanotubes (CNTs) and sodium carboxy methyl cellulose (CMC). The fine voids and bulky structure of p-SiO2 greatly suppress gas and solid heat transfer. The composite of CNT can improve the moldability and enhance the mechanical properties. The moldability of thermal insulating materials improved even with the addition of 1 wt% CNT. With the addition of 1 wt% CNT, the increase in thermal conductivity was less than 0.01 W⋅m-1⋅K-1.

Computation of flow and heat transfer in horizontal gas–solid flows through an adiabatic pipe

The current paper deals with the studies of heat transfer and pressure drop through a horizontal, adiabatic pipe, having gas–solid flows. The inlet air temperature is 443 K, whereas the inlet solid temperature is 308 K. The numerical results are compared with the benchmark experimental data and are agreed satisfactorily. The influences of solid loading ratio, solid diameter and gas velocity on Nusselt number and pressure drop have been studied. The Nusselt number decreases and the pressure drop increases with an increase in the solid diameter. The Nusselt number decreases with an increase in the solid loading ratio at a lower solid diameter of 100 µm. However, at a higher value of solid diameter of 200 µm, the Nusselt number first decreases up to a specific solid loading ratio, and after that, it increases. The pressure drop results show different behaviours with the solid loading ratio. Both the Nusselt number and pressure drop increase with the gas velocity. Finally, a correlation is generated to calculate the two-phase Nusselt number.

Performance of fuel-air combustion in a reheating furnace at different flowrate and inlet conditions

Abstract A geometry model for simulating the gas circulation, heat transfer and slab heating process in a reheating furnace was developed with numerical simulation. The slabs are heated in the furnace and the temperature of the slabs become above more than 1500 K, and then they are transported to the rolling mill. The slab heating in a reheating furnace is influenced by many factors, such as gas velocity, the movement speed of the slab, the height and spacing of the burners and others. The slab heating characteristics in the walking beam type reheating furnace were studied by considering the slab movement. The movement of the slabs was processed by transferring temperature data from the previous location to the next location. It was assumed that no time is spent for moving the slabs. Especially, the effects of different inlet boundaries of the burners were investigated. Results show that the entrainment effect from the inner air flow results in a larger coverage area of the flames, and there is an increase of more than 28.5% in the slab temperature over the whole furnace.

Experimental study of MgO/Mg(OH)2 thermochemical heat storage with direct heat transfer mode

Thermochemical heat storage uses a reversible chemical reaction to store thermal energy. This thermal energy storage method has high energy density and allows long-term thermal energy storage. MgO/Mg(OH)2 is a promising reversible reaction for thermochemical heat storage systems. The performance of this reversible reaction inside a reactor plays a key role in its practical applications. The direct type heat transfer reactor is a novel design compared with an indirect powder bed reactor. It allows the gaseous heat transfer fluid to flow through the powder in order to break the heat transfer limitation from the heat exchanger between the MgO/Mg(OH)2 powder and the heat transfer fluid. In this study, we evaluated the performance of a direct type MgO/Mg(OH)2 reactor under various operating conditions. Parametric analyses indicated that the heat output is limited by the reaction rate rather than the heat transfer efficiency. Results also showed the best bed thickness in order to achieve the highest temperature output. Material characterization indicated that the MgO/Mg(OH)2 particles did not agglomerate with each other after 15 heat charging and discharging cycles.

Numerical study of conduction and radiation heat losses from vacuum annulus in parabolic trough receivers

Parabolic trough receiver is a key component to convert solar energy into thermal energy in the parabolic trough solar system. The heat loss of the receiver has an important influence on the thermal efficiency and the operating cost of the power station. In this paper, conduction and radiation heat losses are analyzed respectively to identify the heat loss mechanism of the receiver. A 2-D heat transfer model is established by using the direct simulation Monte Carlo method for rarefied gas flow and heat transfer within the annulus of the receiver to predict the conduction heat loss caused by residual gases. The numerical results conform to the experimental results, and show the temperature of the glass envelope and heat loss for various conditions in detail. The effects of annulus pressure, gas species, temperature of heat transfer fluid, and annulus size on the conduction and radiation heat losses are systematically analyzed. Besides, the main factors that cause heat loss are analyzed, providing a theoretical basis for guiding the improvement of receiver, as well as the operation and maintenance strategy to reduce heat loss.

Design, space optimization and modelling of solar-cum-biomass hybrid greenhouse crop dryer using flue gas heat transfer pipe network

A solar-cum-biomass hybrid greenhouse crop dryer (HGCD) is presented to work on solar energy and on biomass heat for 24 h continuous operation at constant drying temperature of 62 °C. Vertical gap (clearance) between two consecutive trays is optimized for selected latitudes of 30°, 35°, 40°, 45° and 50°N. Global solar radiation and thermal models are developed to predict the solar radiation availability and HGCD chamber air temperature (Thgcd). Forced draft paddy straw bale combustor (FDPSBC) is used to generate flue gas above 500 °C temperature as supplemental heat source and coupled with flue gas heat transfer pipe network (FGHTPN) laid inside the HGCD to maintain Thgcd at constant temperature. Biomass heating load requirements (forced convection and radiation heats) were predicted through developed heat transfer model. The developed thermal model predicted the Thgcd of 26 °C to 38 °C equivalent to heating load of 4–6.5 kW when ambient air temperature remained between 10 °C and 18 °C at Ludhiana climate (30°N) India. Heat transfer model predicted about 26.2 kW and 13.4 kW of supplemental heat during severe and moderate night temperatures of −5 °C and 10 °C which can be met through complete combustion of 80 kg and 40 kg of paddy straw biomass per hour respectively to maintain Thgcd between 60 and 62 °C. Logarithmic drying model is the best fitting drying model for fenugreek drying. Economic analysis shows that the proposed technology can recover its cost within five years and has great adaptability potential in terms of paddy straw management in India.

A numerical study on gas-to-particle and particle-to-particle heat transfer in a conical fluidized bed reactor

In this present study, the particle behavior and heat transfer coefficients for particle-gas and particle-particle re found by using a computational model in a fluidization process. A conical shape is selected as a reactor due to its wide ange of applications. The equations describing gas and particle motions and heat transfer are solved by using the Eulerian two-fluid approach. Glass bead particles of two different sizes (2 mm and 4 mm) are used as bed materials, and the air is used as a fluidized gas. The velocity of the gas inlet is varied from 1.3 m/s to 2.6 m/s. The results demonstrate that the particle-gas eat transfer coefficient according to the velocity of the air inlet reaches its maximum value around 2.1 m/s, then decreases thereafter. Besides, at the same velocity, the particle-particle heat transfer coefficient reaches its maximum value, then decrease thereafter with increasing the velocity of the inlet air.

An improved model to calculate radiative heat transfer in hot combustion gases

Radiative heat transfer plays an important role in the chemical reactions in the combustor. The widely used WSGG model proposed by Smith is established for normal pressure, which shows inevitable computational errors when dealing with radiative heat transfer problems at reduced or elevated pressures. In this paper, an improved global model is established to calculate the radiant energy exchanges between combustion gases and combustor chamber walls. Compared with the Smith model, the new model shows better performance in a wide range of pressure regions. The model accuracy is examined by computing the emissivity, radiative heat flux as well as the radiative source of H2O–CO2 gas mixtures at different pressure values. Finally, the radiative heat transfer inside a 3D TBCC(turbine-based combined cycle) engine exhaust system where strong gradients of pressure and temperature exist, is also addressed. The computational results show that the developed model provides approximate results at much less computational costs than the high-precision MSMGFSK-c8 model, which makes it competitive in complicated combustion systems.

Influence of burner nozzle configuration, and inlet gas composition on combustion, gas dynamics, temperature and concentration profile in a rotary hearth furnace

ABSTRACT A CDF model for simulating gas dynamics, heat and mass transfer in the freeboard of a pilot-scale Rotary Hearth Furnace (RHF) is developed in conjunction with an independent solid reduction model at the bottom of the hearth. The effect of air-inlet configuration and inlet fuel composition on the fuel requirement, exit gas composition, fluid flow, temperature, and gaseous composition distribution have been investigated using this model. The lower air-inlet tubes orientated at 5° upwards keeping remaining air and fuel tubes horizontal emerged as the most recommended burner configuration. Various alternative fuels categorized in terms of calorific values, demonstrated that the fuel with high calorific value required less fuel and produced less CO2 emission. Interestingly, it was found that for the same calorific value a combination of blast furnace gas and coke oven gas (BF+COG) produced less CO2 emission compared to a combination of COG+BF+NG (Natural gas).

Reactivity Feedback Effect on Supercritical Transient Analysis of Fuel Debris

Transient analysis for possible prompt supercritical accidents of fuel debris in the Fukushima Daiichi Nuclear Power Station is quite important. However, unlike solution fuel systems, there is little knowledge about supercritical transient analysis in fuel debris systems. In particular, reactivity feedback effects, which may have a significant impact on the results of the analysis, are important and require further study. In particular, the impacts of radiolysis gas void and moderator boiling should be discussed. Thus, the purpose of this study is to clarify whether the reactivity feedback effects of radiolysis gas and boiling of the moderator impact the supercritical transient analysis in fuel debris systems. To accomplish this, we used a power profile obtained by the MIK code with the Doppler reactivity feedback effect; radiolysis gas analysis and heat transfer analysis were performed. For the radiolysis gas analysis, the AGNES2 model was modified to consider the difference between solution fuel and fuel debris systems. The heat transfer analysis used an OpenFOAM solver to perform conjugate heat transfer calculations. We found that the radiolysis gas void was negligible when probable G values, which are the generation number of molecules per absorbed energy, were used. In addition, the results showed that boiling could be also negligible under most conditions. However, we found that the boiling time may be earlier than the peak time of the power when the radius of the fuel debris particle is small. In this case, ignoring the boiling may give conservative results. These considerations should be included in future analyses.

Surface coordination of black phosphorene for excellent stability, flame retardancy and thermal conductivity in epoxy resin

Black phosphorus (BP) are shining for its promising properties. Due to the instability and agglomeration problem, the surface coordination strategy is a key point in practical applications. Herein, a ruthenium sulfonate ligand is synthesized to coordinate black phosphorus (BP) nanosheets. By virtue of Ru-P coordination, the lone pair electrons in BP are occupied, thus the RuL3@BP displays excellent stability in environment and different solvents. Subsequently, the resulting RuL3@BP is added into epoxy resin (EP) to fabricate EP nanocomposites. RuL3@BP can effectively enhance the dispersibility of BP in EP due to the surface coordination. When the RuL3@BP is added into epoxy in an amount of 3 wt%, the char yield is distinctly improved by 96.83%, which is ascribed to the cooperative catalytic charring effect between BP and RuL3. EP/RuL3@BP nanocomposites can easily pass the UL-94 V-0 rating, and its limiting oxygen index (LOI) value rises by 26.72%. The peak of heat release rate (PHRR) is decreased by 62.21% and the total heat release (THR) reduces by 35.22%, which is assigned to the restriction of heat transfer and inhibition of flammable gas by the dense char residues. The smoke production and diffusion of thermal pyrolysis gases are dramatically suppressed in the combustion. Meanwhile, owing to strong interfacial interactions between RuL3@BP and EP, EP nanocomposites filled with 3 wt% RuL3@BP exhibit a high thermal conductivity of 0.376 W m−1 K−1, which is enhanced by 52.23% and 65.64% compared with that of EP/BP composite (0.247 W m−1 K−1) and pure EP (0.227 W m−1 K−1), respectively. This surface coordination strategy provides a novel approach for fabricating advanced-performance nanocomposites.