Analysis of local convective heat transfer in a spark ignition engine

A computational fluid dynamics (CFD) code is applied to simulate fluid flow, heat transfer and combustion in a four-stroke single cylinder engine with pent roof combustion chamber geometry, having two inlet valves and two exhaust valves. Heat flux and heat transfer coefficient on the cylinder head, cylinder wall, piston, intake and exhaust valves are determined with respect to crank angle position. Results for a certain condition are compared for total heat transfer coefficient of the cylinder engine with available correlation proposed by experimental measurement in the literature and close agreement are observed. It was found that the local value of heat transfer coefficient varies considerably in different parts of the cylinder, but they have equivalent trend with crank angle. Based on the results, new correlations are suggested to predict maximum and minimum convective heat transfer coefficient in the combustion chamber of a SI engine.     

Numerical study of heat transfer of hydrogen combustion in noble gases atmosphere in compression ignition engine

The high energy content of hydrogen and zero carbon emission from hydrogen combustion is very important for compression ignition engine development. Hydrogen requires a very high auto-ignition temperature, which encourages replacing nitrogen with noble gases with higher specific heat ratio during compression process. In noble gases-hydrogen combustion, higher combustion temperature potentially leading to a higher heat loss. This paper aims to investigate the effect of hydrogen combustion in various noble gases on heat distribution and heat transfer on the cylinder wall. Converge CFD software was used to simulate a Yanmar NF19SK direct injection compression ignition engine. The local heat flux was measured at different locations of cylinder wall and piston head. The heat transfer of hydrogen combustion in various noble gases at different intake temperatures was studied using the numerical approach. As a result, hydrogen combustion in light noble gases such as helium produces faster combustion progress and higher heat temperature. The hydrogen combustion that experienced detonation, which happened in neon at 340 K and argon at 380 K, recorded a very high local heat flux at the cylinder head and piston due to the rapid combustion, which should be avoided in the engine operation. At a higher intake temperature, the rate of heat transfer on the cylinder wall is increased. In conclusion, helium was found as the best working gas for controlling combustion and heat transfer. Overall, the heat transfer data gained in this paper can be used to construct the future engine hydrogen in noble gases.     

The influence of engine speed and load on the heat transfer between gases and in-cylinder walls at fired and motored conditions of an IDI diesel engine

In this study, the heat transfer characteristics between gases and in-cylinder walls at fired and motored conditions in a diesel engine were investigated by using engine data obtained experimentally. For this investigation, a four-cylinder, indirect injection (IDI) diesel engine was tested under different engine speeds and loads. The heat transfer coefficient was calculated by using Woschni expression correlated for the IDI diesel engines, and also using Annand and Hohenberg expressions. The temperature of in-cylinder gases were determined from a basic model based on the first law of thermodynamics after measuring in-cylinder pressure experimentally. The results show that the heat transfer characteristics of the IDI diesel engine strongly depend on the engine speed and load as a function of crank angle at fired and motored conditions.     

Flow and heat transfer characteristics in fuel cooling channels of a recooling cycle

Developing fuel with higher heat sink is widely carried out to meet the cooling requirement for an air breathing hypersonic vehicle. Especially, a recooling cycle has been newly proposed for an actively cooled scramjet to reduce the fuel flow for cooling. Fuel heat sink (cooling capacity) is repeatedly used to indirectly increase the fuel heat sink in a recooling cycle. The variation of fuel thermal property related with heat transfer and flow as temperature and pressure is added to the one-dimensional analytical fin-type model for rectangular ducts. Heat transfer performance comparison between recooling cycle and regenerative cooling is carried out, and detailed discussion about the variation and influence of heat transfer and flow characteristics caused by the introduction of the recooling process are discussed. Performance comparison between a recooling cycle at supercritical pressure and it at subcritical condition is also investigated.     

Free convection heat and mass transfer to steady flow in a semi-infinite vertical porous medium

Analytical solutions are derived for flow in a semi-infinite vertical porous medium with heat and mass transfer. When the temperature and mass concentration are uniform a constant pressure is possible and sustains a fully developed flow. Thereafter there is a small perturbation of the wall temperature and concentration, and the subsequent two-dimensional problem is tackled for large Prandtl number and free convection parameters and small Reynolds number. The heat transfer rate at the wall is discussed quantitatively.     

Performance analysis of organic Rankine cycle based on location of heat transfer pinch point in evaporator

The location of heat transfer pinch point in evaporator is the base of determining operating parameters of organic Rankine cycle (ORC). The physical mathematical model seeking the location of pinch point is established, by which, the temperature variations both of heat source and working fluid with UA can be obtained. Taking heat source with inlet temperature of 160 °C as example, the matching potentials between heat source and working fluid are revealed for subcritical and supercritical cycles with the determined temperature difference of pinch point. Thermal efficiency, exergy efficiency, work output per unit area and maximum work outputs are compared and analyzed based on the locations of heat transfer pinch point either. The results indicate that supercritical ORC has a better performance in thermal efficiency, exergy efficiency and work output while outlet temperature of heat source is low. Otherwise, subcritical performs better. Small heat transfer coefficient results in low value of work output per unit area for supercritical ORC. Introduction of IHX may reduce the optimal evaporating pressure, which has a great influence on heat source outlet temperature and superheat degree. The analysis may benefit the selection of operating parameters and control strategy of ORC.     

The characteristic of polytropic coefficient of compression stroke in hydrogen internal combustion engine

The polytropic coefficient has important effect on the calculation of the instantaneous heat release rate and its comparison with specific heat ratio contains the information of the gas-surroundings heat exchange. This article studies the polytropic coefficient characteristics from 1000 rpm to 5000 rpm and the equivalence ratio from 0.24 to 0.55 by using experimental data from a 2.0 L hydrogen engine. The polytropic coefficient increase from 1.3 to 1.35 with the increase of engine speed from 1000 rpm to 4000 rpm, and then it decrease to around 1.34. This characteristic can be used to calculate the heat release rate more accurately. The study of the effect of equivalence ratio suggested that the polytropic coefficient decreased with the increase of equivalence ratio. The polytropic coefficient of hydrogen engine ranges from 1.28 to 1.35, which is less than the gasoline of 1.32–1.4. And the rising period of polytropic coefficient of ‘hydrogen – 0.55’ at 4500 rpm is very longer than others, which showed that the gases properties had effect on the heat transfer. These characteristics could not only be used for heat transfer calculation, but they can also enrich the research of polytropic coefficient for hydrogen internal combustion engine.     

Thermal and heat transfer characteristics in a latent heat storage system using lauric acid

The thermal and heat transfer characteristics of lauric acid during the melting and solidification processes were determined experimentally in a vertical double pipe energy storage system. In this study, three important subjects were addressed. The first one is temperature distributions and temporal temperature variations in the radial and axial distances in the phase change material (PCM) during phase change processes. The second one is the thermal characteristics of the lauric acid, which include total melting and total solidification times, the nature of heat transfer in melted and solidified PCM and the effect of Reynolds and Stefan numbers as inlet heat transfer fluid (HTF) conditions on the phase transition parameters. The final one is to calculate the heat transfer coefficient and the heat flow rate and also discuss the role of Reynolds and Stefan numbers on the heat transfer parameters. The experimental results proved that the PCM melts and solidifies congruently, and the melting and solidification front moved from the outer wall of the HTF pipe (HTFP) to the inner wall of the PCM container in radial distances as the melting front moved from the top to the bottom of the PCM container in axial distances. However, it was difficult to establish the solidification proceeding at the axial distances in the PCM. Though natural convection in the liquid phase played a dominant role during the melting process due to buoyancy effects, the solidification process was controlled by conduction heat transfer, and it was slowed by the conduction thermal resistance through the solidified layer. The results also indicated that the average heat transfer coefficient and the heat flow rate were affected by varying the Reynolds and Stefan numbers more during the melting process than during the solidification process due to the natural convection effect during the melting process.     

Performance optimization of a solar driven heat engine with finite-rate heat transfer

An optimal performance analysis for an equivalent Carnot-like cycle heat engine of a parabolic-trough direct-steam-generation solar driven Rankine cycle power plant at maximum power and maximum power density conditions is performed. Simultaneous radiation-convection and only radiation heat transfer mechanisms from solar concentrating collector, which is the high temperature thermal reservoir, are considered separately. Heat rejection to the low temperature thermal reservoir is assumed to be convection dominated. Irreversibilities are taken into account through the finite-rate heat transfer between the fixed temperature thermal reservoirs and the internally reversible heat engine. Comparisons proved that the performance of a solar driven Carnot-like heat engine at maximum power density conditions, which receives thermal energy by either radiation-convection or only radiation heat transfer mechanism and rejects its unavailable portion to surroundings by convective heat transfer through heat exchangers, has the characteristics of (1) a solar driven Carnot heat engine at maximum power conditions, having radiation heat transfer at high and convective heat transfer at low temperature heat exchangers respectively, as the allocation parameter takes small values, and of (2) a Carnot heat engine at maximum power density conditions, having convective heat transfer at both heat exchangers, as the allocation parameter takes large values. Comprehensive discussions on the effect of heat transfer mechanisms are provided.     

Modelling heat transfer in a circulating fluidized bed

A model for the bed-to-wall heat transfer under low temperature condition in a circulating fluidized bed (CFB) was developed based upon a simplified cluster renewal concept. The age of clusters in contact with the wall at different locations along the height of the CFB was estimated as the weighted average age considering their formation and disintegration. One set of experimental data on heat transfer in a 4.5-metre high, 0.15-metre diameter CFB under low temperature condition (67–77°C) was chosen for comparison with prediction of local heat transfer coefficient. The experimental observation and prediction have shown a qualitative agreement.     

Thermalhydraulic analysis and heat transfer correlation for an intermediate heat exchanger linking a SuperCritical Water-cooled Reactor and a Copper-Chlorine cycle for hydrogen co-generation

This paper presents the development of a new heat-transfer correlation for the flow of SuperCritical Water (SCW) in bare circular tubes, and its applicability to thermalhydraulic analysis for Heat eXchanger (HX) designs, linking a SuperCritical Water-cooled nuclear Reactor (SCWR) and a Copper-Chlorine (Cu–Cl) cycle-based hydrogen co-generation facility.     

Simultaneous heat and mass transfer on oscillatory free convection boundary layer flow

The problem of simultaneous heat and mass transfer in two-dimensional free convection from a semi-infinite vertical flat plate is investigated. An integral method is used to find a solution for zero wall velocity and for a mass transfer velocity at the wall with small-amplitude oscillatory wall temperature. Low- and high-frequency solutions are developed separately and are discussed graphically with the effects of the parameters Gr (the Grashof number for heat transfer), Gc (the Grashof number for mass transfer) and Sc (the Schmidt number) for Pr = 0–71 representing aid at 20°C.     

Laminar heat transfer in a porous channel simulated with a two-energy equation model

Laminar heat transfer in a porous channel is numerically simulated with a two-energy equation model for conduction and convection. Macroscopic equations for continuity, momentum and energy transport for the fluid and solid phases are presented. The numerical methodology employed is based on the control volume approach with a boundary-fitted non-orthogonal coordinate system. Fully developed forced convection in a porous channel bounded by parallel plates is considered. Solutions for Nusselt numbers along the channel are presented for laminar flows. Results simulate the effects Reynolds number Re, porosity, particle size and solid-to-fluid thermal conductivity ratio on Nusselt sumber, Nu, which is defined for both the solid and fluid phases. High Re, low porosities, low particle diameters and low thermal conductivity ratios promote thermal equilibrium between phases leading to higher values of Nu.     

Effects of porosity on heat and mass transfer in a granular adsorbent bed

In the present study, the mechanism of heat and mass transfer in an annulus adsorbent is handled. The heat and mass transfer equations for the adsorbent bed and the mass balance equation for the adsorbent granules are numerically solved to obtain the distributions of temperature, pressure, adsorptive density and adsorbate concentration in the adsorbent bed. The study is performed for the silica gel–water pair and for three different values of porosity as 0.1, 0.2 and 0.3. The distributions of temperature and adsorbate concentration are considerably influenced from the bed porosity. The adsorption period increases with the increase of the porosity value. The porosity affects the pressure and adsorptive density distributions at the beginning of the process and after a relatively short time, the averages of these dependent variables approach to the final equilibrium state.     

Study of the compression cycle of a reciprocating engine through the polytropic coefficient

The polytropic coefficient during the compression cycle of a reciprocating IC engine depends on the instantaneous values for pressure and volume, as well as on their variations. This dependency makes it useful to solve uncertainties typical of the engine experimental tests, such as the synchronization of pressure and volume signals. Additionally, knowledge of the polytropic coefficient is useful if the heat flux transferred to the walls is to be calculated, this variable being difficult to obtain either from modelling or from measurements, or to minimize errors on the estimation of geometric parameters such as the compression ratio.     

Performance of a combined organic Rankine cycle and vapor compression cycle for heat activated cooling

Heat activated cooling has the potential of utilizing thermal sources that currently go unused such as engine exhaust heat or industrial waste heat. Using these heat sources can provide enhanced energy utilization and reduced fuel usage in applications where cooling is needed. The concept developed here uses waste heat from stationary and mobile engine cycles to generate cooling for structures and vehicles. It combines an organic Rankine cycle (ORC) with a conventional vapor compression cycle. A nominal 5 kW cooling capacity prototype system was developed based on this concept and tested under laboratory conditions. In order to maintain high system performance while reducing size and weight for portable applications, microchannel based heat transfer components and scroll based expansion and compression were used. Although the system was tested off of its design point, it performed well achieving 4.4 kW of cooling at a measured heat activated COP of 0.48. Both the conversion and 2nd law efficiencies were close to the model results, proving it to be an attractive technology. The measured isentropic efficiency of the scroll expander reached 84%, when the pressure ratio was close to the scroll intrinsic expansion ratio. The reduced cooling capacity was attributed to off design operation.     

The use of phase-change coatings for estimating the heat transfer characteristics in a rotating disc system

Phase-change coatings have been applied to the axial-clearance rotor-stator cavity for the estimation of the transient heat transfer characteristics of the surface of the rotating disc. The tests were conducted for an air mass flow coefficient C_w = 1220, a gap ratio G = 0.1, an axial-clearance ratio G_ca = 0.05 and for rotational Reynolds numbers of Re? = 1 × 10⁵ and 2 × 10⁵. The phase-change coating used had a melting point of 38°C. From a video recording of the transient movement of the melt-line on the rotor (coated with the phase-change material) blown with heated air, it was possible to compute the heat transfer coefficients. The data reduction was made using the ‘semi-infinite slab’ approximation to the governing one-dimensional transient heat conduction equation.     

Experimental studies on the behaviours of hydride heat storage system

Experimental studies on the heat transfer characteristics of the heat storage vessel are conducted. Mg₂Ni is used as the heat storage medium. The structure of the heat storage vessel is a single tube in which a number of thermocouples are installed for temperature measurement. The temperature gradient along the axial direction of the tube was not found at the steady state for both heat generation and absorption period. Temperature gradient, however, exists along the radial direction. The overall heat transfer coefficient obtained is about 10 Kcal/m²h deg which implies that heat transfer through the powder bed is a controlling step. The lumped parameter model to describe the performance of the heat storage vessel was derived and it could explain the heat transfer characteristics of the vessel qualitatively.     

Experimental study on heat and mass transfer characteristics of louvered fin-tube heat exchangers under wet condition

An experimental study was conducted to investigate the effect of a tube row, a fin pitch and an inlet humidity on air-side heat and mass transfer performance of louvered fin-tube heat exchangers under wet condition. Experimental conditions were varied by three fin pitches, two rows, and two inlet relative humidities. From the experimental results, it was found that the heat transfer performance decreased and the friction increased with the decrease of a fin pitch, for 2 row heat exchanger. The effect of a fin pitch on heat transfer performance was negligible with 3 row heat exchanger. The change in a relative humidity was not affected heat transfer and friction. However, the mass transfer performance was slightly decreased with the increase of a relative humidity and with the decrease of a fin pitch. The mass transfer performance decreased with the decrease of a fin pitch. The mass transfer performance of the louvered fin-and-tube heat exchanger was different according to the number of a tube row.