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This paper conducts an investigation on the temperature variations experienced by the fuel when it expands through the calibrated orifices of a commercial diesel injector. Experimental results of the temperature change across a calibrated orifice upon expansion, extracted from a previous work, are compared to the temperature predicted by computational fluid dynamic simulations under the assumption of adiabatic flow, with no heat transfer to the surroundings. The comparison points out that the simulations are able to predict the thermal effects taking place inside the orifice. Once the model is validated, the flow morphology is analyzed to explain the trends observed in the fuel temperature change across the orifice depending on the operating conditions. Two opposed effects take place inside the orifice: on the one hand, the flow is cooled in the orifice core due to depressurization; on the other hand, the fuel is importantly heated near the walls due to viscous friction. As expected, the net effect on the outlet temperature mainly depends on the orifice discharge coefficient, governed by the orifice geometry and the flow regime (Reynolds number) induced by the injection conditions. Next, the analysis is extended to a diesel nozzle, considering that the higher pressure drops achieved in it are expected to induce even more important thermal effects. The two opposed effects also take place inside the orifice. Even though their net effect is similar, the separate effect of each phenomenon is greater, leading to differences that could be relevant for the atomization and spray formation processes. Additionally, the flow pattern shows a non-uniform distribution of the flow inside the nozzle influencing the results from the thermal point of view. [ABSTRACT FROM AUTHOR]

• A methodology to evaluate residuals temperature and composition effects is proven. • Internal residuals provide faster kernel growth and shorter combustion duration. • Composition of internal residuals has a positive combustion impact at high rates. • Internal residuals imply higher heat transfer, compensating combustion benefits. • Correlations for combustion timings vs. residuals characteristics are developed. Several technologies are being put in place to improve the efficiency of spark-ignition engines. Two of these technologies are variable valve actuation and exhaust gas recirculation, both of which can have an impact on combustion according to the literature. However, this impact is due to variations in both the in-cylinder temperature and composition, linked to the different characteristics of internal and external residuals, which are difficult to decouple. In this paper, a methodology aimed at improving the understanding of the temperature and composition effects on combustion parameters is developed. Internal residuals are controlled through a variable valve timing actuation, and evaluated thanks to a previously validated one-dimensional engine model, while the externally-circulated exhaust are extracted after the catalyst through a low-pressure system. Combustion is analyzed based on in-cylinder pressure acquisition and emissions measurement at the engine-out, before the aftertreatment module. Different combinations of internal and external residuals are tested with different strategies from the point of view of the spark actuation: first, a constant spark timing for each value of total residuals; then, a spark timing adapted to maintain the same temperature inside the cylinder at the combustion start, and finally an optimized calibration from the point of view of the engine efficiency. Results show a general trend to enhance combustion timings as higher share of internal residuals are introduced for the same amount of total residuals. It can be seen how this trend comes mostly from the temperature effect at low total residuals fraction, while composition effects are more significant as the amount of total residuals increase. Empirical correlations developed in the paper confirm the need to decouple internal and external residuals to have a proper estimation of the combustion timings. Despite the positive impact of internal residuals on the combustion duration, a small fuel consumption benefit is reached with cooled exhaust gas recirculation thanks to lower heat transfer losses. In terms of emissions, unburned hydrocarbon emissions tend to be reduced with increasing the share of internal residuals thanks to the higher combustion speed, while other gaseous emissions are mostly affected by the total residuals amount. Therefore, internal and external residuals could be combined to achieve the best trade-off between fuel consumption and hydrocarbon emissions. [ABSTRACT FROM AUTHOR]

Abstract: The non-insulated and insulated ducts are commonly applied in the industries and various buildings, because the heat radiation equation contains the 4th order exponential of temperature which is very complicate in calculations. Most heat transfer experts recognized from their own experiences that the heat radiation effect can be ignored due to the small temperature difference between insulated and non-insulated surface and surroundings. This paper studies in detail to check the inaccuracies of heat transfer characteristics non-insulated and insulated duct by comparing the results between considering and neglecting heat radiation effect. It is found that neglecting the heat radiation effect is likely to produce large errors of non-insulated and thin-insulated ducts in situations of ambient air with low external convection heat coefficients and larger surface emissivity, especially while the ambient air temperature is different from that of surroundings and greater internal fluid convection coefficients. It is also found in this paper that using greater duct surface emissivity can greatly improve the heat exchanger effect and using smaller insulated surface emissivity can obtain better insulation. [Copyright &y& Elsevier]

Inverse methods have many applications in determining unknown variables in heat transfer problems when direct measurements are impossible. As most common inverse methods are iterative and time consuming especially for complex geometries, developing more efficient methods seems necessary. In this paper, a direct transient conduction–convection heat transfer problem (fin) under several boundary conditions was solved by using lattice Boltzmann method (LBM), and then the results were successfully validated against both the finite difference method and analytical solution. Then, in the inverse problem both unknown base temperatures and heat fluxes in the rectangular fin were estimated by combining the adjoint conjugate gradient method (ACGM) and LBM. A close agreement between the exact values and estimated results confirmed the validity and accuracy of the ACGM–LBM. To compare the calculation time of ACGM–LBM, the inverse problem was solved by implicit finite difference methods as well. This comparison proved that the ACGM–LBM was an accurate and fast method to determine unknown thermal boundary conditions in transient conduction–convection heat transfer problems. The findings can efficiently determine the unknown variables in fins when a desired temperature distribution is available. [ABSTRACT FROM AUTHOR]

This paper investigates the effect of naphtha (RON = 65.6) and commercial gasoline (RON = 94.0) on Multiple Premixed Compression Ignition (MPCI) mode. The experiment is conducted on a single cylinder research diesel engine with compression ratio of 16.7. The engine is operated at an engine speed of 1600 rpm for the IMEP from 0.4 to 1.4 MPa. Commercial diesel (CN = 56.5) is also tested in Conventional Diesel Combustion (CDC) mode as a baseline. At each operating point, the injection strategy and intake conditions are adjusted to meet with the criteria (NO x < 0.4 g/kW h, soot < 0.06 m −1 , MPRR < 1 MPa/deg and CA50 < 20 CAD ATDC). The typical two-stage combustion characteristics of MPCI are obtained in both naphtha and gasoline. Stable combustion is achieved by naphtha in wide load range, while the engine fuelled with gasoline cannot operate stably at 0.4 MPa IMEP. The COV of IMEP of gasoline MPCI is higher than that of naphtha and diesel. However, gasoline has the low MPRR and the retarded CA50 at medium and high loads due to its longest ignition delay. As a result of low exhaust loss for naphtha and low heat transfer loss for gasoline, the thermal efficiencies are higher for both naphtha and gasoline in MPCI mode than diesel in CDC mode, even though diesel has the highest combustion efficiency. The separated combustion in MPCI leads to low cylinder temperature, and moderate EGR ratio (less than 30%) is needed to control NO x emissions under the limit of EURO VI. [ABSTRACT FROM AUTHOR]

In this paper a further development of methodology for decreasing the capital cost for Total Site heat recovery by use of different utility levels is proposed. The capital cost of heat recovery system is estimated for certain temperature level of intermediate utility applying Total Site Profiles. Heat transfer area is reduced by selection of appropriate temperature of intermediate utility. Minimum of heat transfer area depends on slopes of Total Site Profiles in each enthalpy interval. This approach allows estimating the minimum of heat transfer area for heat recovery on Total Site level. Case study is performed for fixed film heat transfer coefficients of process streams and intermediate utilities. It indicates that the total heat transfer area of heat recovery can be different up to 49.15% for different utility temperatures. [ABSTRACT FROM AUTHOR]

Abstract: If the temperature change from the base to the tip of the fin is large, then for an accurate prediction of the performance of the fin, the model must incorporate the variation of thermal conductivity with temperature. Also, the heat exchange with the surrounding air is associated with the heat transfer coefficient. It has been experimentally proven that the heat transfer coefficient may not be uniform and there are large differences in the heat transfer coefficient both from fin base to fin tip and along the fin in the direction of the airflow. Same as two parameters mentioned above, the surface emissivity and internal heat generation within the fins may vary with temperature. This paper analytically investigates the temperature distribution within convective–radiative straight fins with simultaneous variation of thermal conductivity, heat transfer coefficient, surface emissivity and internal heat generation with temperature. Also, convection and radiation sink temperatures were assumed to be non-zero. Four different longitudinal fins such as rectangular, triangular, convex and exponential were considered. The calculations are carried out using the well-known approximate analytical differential transformation technique. Fin efficiency and the effects of some physically applicable parameters in this problem such as thermal conductivity, convection–conduction parameter, radiation–conduction parameter, convection sink temperature and radiation sink temperature were studied comparatively. Because a broad range of governing parameters are investigated, the results should be useful in a number of engineering applications. [Copyright &y& Elsevier]

Abstract: Loop heat pipe (LHP) is a promising means for electronics cooling since LHP is a exceptionally efficient heat transfer device. In this paper, a novel miniature LHP (mLHP) system is presented and optimal design of condenser is considered seeing that evaporators have been able to handle very high-heat fluxes with low-heat transfer resistances since most of the previous researchers focused on the evaporator of mLHP. The arrayed pins were designed and machined out on the bottom of condenser to enhance condensation heat transfer. The parameters of the arrayed pins, including layout, cross-section shape and area, were optimized by finite element analysis. Tests were carried out on the mLHP with a CPU thermal simulator using forced air convection condenser cooling to validate the optimization. The operation characteristics of the mLHP with optimal design parameters of condenser were investigated experimentally. The experimental results show that the mLHP can reject head load 200W while maintaining the cooled object temperatures below 100°C, and for a variable power applied to the evaporator, the system presents reliable startups and continuous operation. [Copyright &y& Elsevier]

Abstract: This paper numerically investigates the performance of composite spreaders comprised of Cu substrates and Cu micro-porous surfaces of different thicknesses for immersion cooling of 10×10mm underlying computer chip with a 2×2mm central hot spot. The local heat flux at the hot spot is three times the chip’s surface average outside the hot spot. The thickness of the Cu substrate changes from 1.6 to 3.2mm and that of the Cu micro-porous surface changes from 80 to 197μm. The spreaders are cooled by saturation nucleate boiling of PF-5060 dielectric liquid. The local values of the nucleate boiling heat transfer coefficients on the various Cu micro-porous surfaces are based on pool boiling experimental measurements. Results demonstrated the effectiveness of immersion cooling nucleate boiling for mitigating the effect of the hot spot. The spreaders decrease the maximum surface temperature and the temperature gradient on the chip surface and increase the dissipated thermal power by the chip and removed from the spreader surface. Increasing the thickness of the Cu substrate and/or decreasing the thickness of the Cu micro-porous surface increases the total thermal power removed, the chip surface temperature and the spreader’s footprint area. [Copyright &y& Elsevier]

Abstract: Compared to the pure refrigerants, the zeotropic refrigerant mixtures have the obvious temperature glide during phase change. Therefore, the Lorenz cycle can be approached with this special attribute. By analysing the heat transfer in the counter flow heat exchanger, a new evaluation method for zeotropic refrigerant mixtures based on the variance of the temperature difference between the refrigerant and heat transfer fluid (HTF) is proposed in this paper. For approaching to the Lorenz cycle and perfect glide matching, the zeotropic mixture which has smaller variance in the heat exchanger should be chosen in the refrigeration cycle. The variance of temperature difference is affected by two factors which are the temperature difference between the inlet and outlet of HTF and the linear relationship between the refrigerant temperature and enthalpy, respectively. The smallest variance of the zeotropic refrigerant can be obtained by setting the temperature difference of HTF to be the optimal temperature difference. [ABSTRACT FROM AUTHOR]

Abstract: Analytical solutions for evaluating the thermal performances of both chilled water wet cooling coils and direct expansion (DX) wet cooling coils, respectively, under both unit and non-unit Lewis Factors are developed and reported in this paper. The analytical solution was validated by comparing its predictions with those from numerically solving the fundamental governing equations of heat and mass transfer taking place in a wet cooling coil. With the analytical solutions, the distributions of air temperature and humidity ratio along air flow direction in a wet cooling coil can be predicted, and the differences in the thermal performances of the cooling coils under both unit and non-unit Lewis Factors can be identified. The analytical solutions, on one hand, can be a low-cost replacement to numerically solving the fundamental heat and mass transfer governing equations, and on the other hand, is able to deal with evaluating thermal performance for wet air cooling coils operated under both unit and non-unit Lewis Factors. [Copyright &y& Elsevier]

Abstract: Heat pipe with groove--shaped capillary structure always shows smaller heat transfer limit. It is because that traditional fabrication technique leads to the difficulty in minimizing the geometric dimension of grooves. As a result, novel fabrication technique is of great importance. In this paper, we will discuss some heat transfer characteristics of heat pipe fabricated by novel Extrusion–ploughing process. Owing to the extrusion from tools in the fabrication course, two kinds of grooves, namely leading grooves and secondary grooves, are formed in this kind of heat pipe. This feature will polish the groove geometric structure. Furthermore, considering the fact of parallel combination of these two kinds of grooves, excellent heat transfer performance of novel heat pipe fabricated by novel Extrusion–ploughing process can be expected. The experiments of testing temperature changes versus time at evaporator sector and condenser sector at different work temperature are conducted. The temperature distribution along axial direction of this kind of novel heat pipe at its steady stage is also tested. The experimental results prove that the heat pipe fabricated by Extrusion–ploughing process exhibits ideal isothermal performance and excellent heat respond speed. When compared with normal axially grooved heat pipe, this novel kind of heat pipe also shows larger final heat transfer limit. [Copyright &y& Elsevier]

Abstract: Theoretical investigations on the application of nanofluids in the hydronic heating system of buildings have been presented in this paper. Realistic fluid velocities and temperatures employed in heat exchange equipment were used to compare the convective heat transfer coefficient and the pumping power requirement of nanofluids of various particle volume concentrations. The base fluid used in the analyses as a basis for comparison was a solution comprised of 60% ethylene glycol and 40% water (by weight). The thermophysical and heat transfer performance of the base fluid have been compared with copper oxide nanoparticles dispersed in the same base fluid with volumetric particle concentration ranging up to 4%. First, the variations of thermophysical properties such as viscosity, thermal conductivity, specific heat and density have been evaluated for different concentrations. Using these properties, several plots have been developed for quantitatively determining the enhancement of convective heat transfer coefficient as a function of flow velocity, Reynolds number and the nanoparticle volumetric concentration. For constant Reynolds number for flow in tubes, the Nusselt number and convective heat transfer coefficient increases as nanoparticle volume concentration increases. As the properties of nanofluids and the base fluid are temperature dependent, we have performed calculations for two fluid temperatures (323K and 360K). The results show that a higher convective heat transfer coefficient is achieved at higher temperatures for a nanofluid of equal concentration circulating at equal velocity. Frictional pressure loss and pumping power increases relative to the base fluid with particle volume concentration, but decreases with mean fluid temperature. [Copyright &y& Elsevier]