The early flame kernel initiation and development are essential to a successful combustion process, especially under lean burn/EGR diluted conditions. Multiple ignition sites strategy has shown promise to secure the flame kernel initiation under extreme engine operating conditions. Two factors are considered to contribute to the enhanced ignition capability, i.e. the higher ignition energy and the multiple initial flame kernels. However, the mechanism why the multiple ignition sites help combustion is less understood. In this work, the impacts of the ignition energy distribution strategy on the flame inception process are investigated in a constant volume combustion chamber. A multi-coil ignition system, along with a sparkplug with three high-voltage electrodes, is used to adjust the discharge energy from 10 mJ to 240 mJ, as well as the energy deposition strategies. Experimental results have shown that the distributed energy strategy with sufficient discharge energy can establish a bigger initial flame kernel, leading to faster flame growth rates, as compared to the concentrated energy strategy.

In this paper, the electrical-to-thermal energy transfer efficiency of the transistor coil ignition system for spark-ignition engines is investigated using both electrical and calorimetry measurements. The gap voltage and discharge current are measured to determine the electrical energy supplied to the spark gap. A pressure-rise calorimeter is used to estimate the thermal energy transferred from the plasma channel to the gas. Firstly, this paper studies the influences of spark gap size, electrode geometry and background pressure on the energy transfer efficiency. To further investigate the effectiveness of increasing breakdown energy on the energy transfer process, a direct-capacitor is paralleled to the spark gap to redistribute the spark energy in both breakdown and glow phases. The varying of the capacitance enables the investigation of the energy transfer efficiency under different breakdown energy level. Results show that the electrical-to-thermal energy transfer efficiency is strongly dependent on gap size, electrode geometry and background pressure. Increasing the breakdown discharge energy is beneficial for the electrical to thermal energy transfer process.

An experimental study of saturated flow boiling in a high-aspect-ratio one-side-heating rectangular microchannel is conducted with deionized water as the working fluid. The bottom surface of the microchannel is constructed of two different material, one is the untreated hydrophilic silicon wafer with a contact angle of 65° ± 3°, the other is the super-hydrophilic silicon wafer deposited with a thin film of 100-nm-thickness silicon dioxide through PECVD with a contact angle less than 5°. The heat transfer characteristics of saturated flow boiling in the microchannel is studied and the flow pattern is photographed with a high speed camera. It is found that the heat transfer mechanism is dominated by convective evaporation, the heat flux transfers through the conduction and convection in the thin liquid film and evaporation at the interface between the vapor and liquid. The thinner the liquid film, the greater the heat transfer coefficient. The local dryout phenomenon is observed on the untreated hydrophilic surface while the super-hydrophilic surface can restrict the occurrence of local dryout phenomenon. This study proposes the modified Li (2010) correlation as a formula for calculating the heat transfer coefficient.

For improving the functionality and signal speed of electronic devices, electronic components have been miniaturized and an increasing number of elements have been packaged in the device. As a result there has been a steady rise in the amount of heat necessitated to be dissipated from the electronic device. Recently microchannel heat sinks have been emerged as a kind of high performance cooling scheme to meet the heat dissipation requirement of electronics packaging, In the present study an experimental study of subcooled flow boiling in a high-aspect-ratio, one-sided heating rectangular microchannel with gap depth of 0.52 mm and width of 5 mm was conducted with deionized water as the working fluid. In the experimental operations, the mass flux was varied from 200 to 400 kg/m 2 s and imposed heat flux from 3 to 20 W/cm 2 while the fluid inlet temperature was regulated constantly at 90 °C. The boiling curves, flow pattern and onset of nucleate boiling of subcooled flow boiling were investigated through instrumental measurements and a high speed camera. It was found that the slope of the boiling curves increased sharply once the superheat needed to initiate the onset of nucleate boiling was attained, and the slope was greater for lower mass fluxes, with lower superheat required for boiling incipience. As for the visualization images, for relatively lower mass fluxes the bubbles generated were larger and not easy to depart from the vertical upward placed narrow microchannel wall, giving elongated bubbly flow and reverse backflow. The thin film evaporation mechanism dominated the entire test section due to the elongated bubbles and transient local dryout as well as rewetting occurred. Meanwhile the initiative superheat and heat flux of onset of nucleate boiling were compared with existing correlations in the literature with good agreement.

The flow field inside the heat exchangers is associated with maximum heat transfer and minimum pressure drop. Designing a compact heat exchanger and employing various techniques to enhance its overall performance has been widely investigated and still an active research field. However, few researches deal with thermal optimization. The application of elliptic tube is an effective alternative to circular tube which can reduce the pressure drop significantly. In this study, numerical simulation and optimization of variable tube ellipticity is studied at low Reynolds numbers. The three-dimensional numerical analysis and a multi-objective genetic algorithm (MOGA) with surrogate modelling is performed. Two row tubes in staggered arrangement in fin-and-tube heat exchanger is investigated for combination of various elliptic ratio ( e = minor axis/major axis) and Reynolds number. Tube elliptic ratio ranges from 0.2 to 1 and Reynolds number ranges from 150 to 750. The tube perimeters are kept constant while changing the elliptic ratio. The numerical model is derived based on continuum flow approach and steady-state conservation equations of mass, momentum and energy. The flow is assumed as incompressible and laminar due to low inlet velocity. Results are presented in the form of Colburn factor, friction factor, temperature contours and streamline contours. Results show that increasing elliptic ratio increases the friction factor due increased flow blocking area, however, the effect on the Colburn factor is not significant. Moreover, tube with lower elliptic ratio followed by higher elliptic ratio tube has better thermal-hydraulic performance. To achieve maximum heat transfer enhancement and minimum pressure drop, the Pareto optimal strategy is adopted for which the CFD results, Artificial neural network (ANN) and MOGA are combined. The tubes elliptic ratio (0.2 ⩽ e ⩽ 1.0) and Reynolds number (150 ⩽ Re ⩽ 750) are the design variables. The objective functions include Colburn factor ( j ) and friction factor ( f ). The CFD results are input into ANN model. Once the ANN is computed and its accuracy is checked, it is then used to estimate the model responses as a function of inputs. The final trained ANN is then used to drive the MOGA to obtain the Pareto optimal solution. The optimal values of these parameters are finally presented.

Experimental condensation heat transfer data from our previous studies were collected to find a suitable heat transfer correlation for three-dimensional surface enhanced heat transfer tubes. Dimples/protrusions and petal arrays make up the enhanced surface of the 1EHT tube, while longitudinal grooves and dimples constitute that of the other two 2EHT tubes. Working fluids investigated includes three refrigerants R22, R32, R410A and three enhanced tubes have a same nominal inner diameter of 12.7mm. Due to the unique and complex surface structure of three EHT tubes, a constant is proposed and utilized for each EHT tube to estimate the impact of enhanced surfaces on interfacial turbulence, boundary layer disruption, flow separation and secondary flow generation. In addition, the Cavallini correlation modified with the constant can predict almost all the data points within a ±5% error band, which can be a good example for how to predetermine the heat transfer coefficient of three-dimensional enhanced tubes with a unique surface.

An experimental study on local flow boiling heat transfer performance in two longitudinal dimple-grooved tubes and an equivalent smooth tube was performed. All three test cooper tubes have the same inner diameter of 11.5 mm; the working fluid is the near-azeotropic mixture, R410A; and all test runs are conducted in a 2 m long horizontal tube-in-tube heat exchanger. Constant evaporation temperature at 10 °C was maintained when heat flux ranging from 32 kW/m 2 to 37 kW/m 2 and refrigerant quality varied from inlet 0.1 to outlet 0.9 at mass flux 150 kg/(m 2 s). The local heat transfer coefficients were obtained for all test conditions using refrigerant R410A. The test results for evaporation were presented compared to the equivalent smooth tube. Wall temperature and local and average heat flux is measured; heat flux effect and surface superheat effect is discussed on the tube side evaporation. The enhanced heat transfer area of two longitudinal dimple-grooved tubes are 1.02 and 1.03, respectively.

Enhanced condensation heat transfer of two-phase flow on the horizontal tube side receives more and more concerns for its fundamentality and importance. Experimental investigations on convective condensation were performed respectively in different horizontal tubes: (i) a smooth tube (11.43 mm, inner diameter); (ii) a herringbone tube (11.43 mm, fin root diameter); and (iii) three enhanced surface (EHT) tubes (11.5 mm, equivalent inner diameter): 1EHT tube, 2EHT-1 tube and 2EHT-2 tubes. The surface of EHT tubes is enhanced by arrays of dimples with the background of petal arrays. Experiments were conducted at a saturation temperature of approximately 320 K; 0.8 inlet quality; and 0.2 outlet quality; 72–181 kg·m −2 ·s −1 mass flux using R22, R32 and R410A as the working fluid. The refrigerant R32 presents great heat transfer performance than R410A and R22 at low mass flux due to its higher latent heat of vaporization and larger thermal conductivity. The heat enhancement ratio of the herringbone tube is 2.72–2.82, rated number one. The primary dimples on the EHT tube increase turbulence and flow separation, and the secondary petal pattern produce boundary layer disruption to many smaller scale eddies. The 2EHT tubes are inferior to the 1EHT tube. A performance factor is used to evaluate the enhancement effect except of the contribution of area increase.

Supercritical fluids are widely used in aeronautic, astronautic and nuclear engineering. Active cooling is necessary for scramjet engines to survive the extreme heat generated in hypersonic flight. Regenerative cooling system, where engine fuel works as coolants and travels through the cooling tubes along the chamber wall, carrying away heat from the wall via heat convection and endothermic chemical reactions, is developed as an effective thermal management technique. In this paper, experimental results of convective heat transfer performances of aviation kerosene at supercritical pressures were presented. Stainless steel circular tubes having inner diameters of 1and 1.8 mm were investigated for pressures ranging from 3 to 4 MPa, mass flow rates from 1.87 to 2.41 g/s and heat fluxes from 285 to 365 kW/m 2 . It was found that the heat transfer coefficient increases with mass flow rate at the former part of the tube. However, as the Reynolds increases significantly at the latter part of the tube at relatively low mass flow rate, the heat transfer coefficient increases dramatically at the latter part of the tube at relatively low mass flow rate. The effect of heat flux on heat transfer is complicated, while the effect of pressure on heat transfer is insignificant. The experimental results also indicated that the heat transfer coefficient decreases with the reduction in tube diameter. The heat transfer behaviors in relation to changes in tube sizes might be caused by the buoyancy effect.