Li Guoneng

Convective Heat Transfer Enhancement of a Rectangular Flat Plate by an Impinging Jet in Cross Flow

Abstract Experiments were carried out to study the heat transfer performance of an impinging jet in a cross flow. Several parameters including the jet-to-cross-flow mass ratio ( X 2%–8%), the Reynolds number ( Re d 1434–5735) and the jet diameter ( d 2–4 mm) were explored. The heat transfer enhancement factor was found to increase with the jet-to-cross-flow mass ratio and the Reynolds number, but decrease with the jet diameter when other parameters maintain fixed. The presence of a cross flow was observed to degrade the heat transfer performance in respect to the effect of impinging jet to the target surface only. In addition, an impinging jet was confirmed to be capable of enhancing the heat transfer process in considerable amplitude even though the jet was not designed to impinge on the target surface.

Experimental study on a potable thermoelectric power generating stove

Electricity is not available everywhere, and a significant portion of people still lives without electricity. Available commercial potable thermoelectric power generating stoves are rare. In order to obtain electrical energy in off-grid areas and in special conditions (earthquake, hurricane, tidal wave, military field, etc.), a potable thermoelectric power generating stove with eight thermoelectric modules was designed. The starting-up performance, temperature level, power load feature and thermoelectric efficiency were tested experimentally using thermocouples, electronic load and corresponding data acquisition systems. Biomass fuel, e.g. dry branches and withered-grasses, can be used in the thermoelectric power generating stove. The weight and the dimensions of the stove are 2.5 kg and 0.25 m×0.2 m×0.044 m (after folded), respectively. No battery is embedded inside, and the stove can producing electricity continuously in case of fire up. The fans for the heat sink are self-starting within 2 min after fire up, and electrical energy can be extracted from the stove to the outside electric equipment subsequently. Tested results found that a maximum output power of 2.45 W at 12.2 V can be obtained after self-powered fans for the heat sink when the load resistance lies between 50 Ω and 70 Ω . The system will fail to produce electricity when the load resistance is smaller than 40 Ω , while the output power decreases when the load resistance continues to increase from 70 Ω . In case a voltage converter with a conversion efficiency of 84% was adopted to maintain a 5.0 V output voltage, a maximum output power of 2.06 W was recorded when the load resistance is 12 Ω . Smaller load resistance will cause system failure, and larger load resistance results in less power output. The thermoelectric efficiency was found to be about 2.1% for the present stove based on experimental measurements and one dimensional Fourier law approximation. On the other hand, it is found that the working temperature difference is about 66°C, indicating that a large potential to increase the power output exists, and convective heat transfer enhancing methods for the heat sink should be a possible solution. This study reveals that the heat management is the key principle in the thermoelectric power generating technology. The zero energy consumption design of the heat end, the temperature controlling strategy for the heat end, the total heat capacity of the heat sink for the fan self-starting, the electrical power load selection for the cold end are the key parameters to extract electrical energy from the system. Comparisons were made between present results and tested data of a commercial potable thermoelectric power generating stove, and analysis and discussions were made. The tested commercial one has a weight of 0.934 kg, and producing a maximum output power of 1.07 W continuously when the load resistance is 5 Ω under the working temperature difference of about 72°C. It is showed that the ratio of output power to weight of the present potable thermoelectric power generating stove is 1.0 W/kg, while the commercial stove is 1.14 W/kg. The ratio of output power to volume for the present stove is 936 W/m3, while the commercial one is 540 W/m3.

The Mixing Characteristics of a Transverse Jet under Different Reynolds Number and Velocity Ratio

In order to study the mixing characteristics of a lateral jet issuing into crossflow, a flow-visualization based experimental setup was built. A phase tunable laser and CCD system was employed to investigate the structure of the flow field of a lateral jet issuing into crossflow under different jet to crossflow velocity ratio and under different Reynolds number. The central trajectory and the spread width are obtained through the calculation of the matrix of the gray data. Experimental results showed that the larger jet to crossflow velocity ratio resulted in deeper penetration depth, yet it is not the greater access to the spread width, but there is an optimized jet to crossflow velocity ratio to obtain the largest spread width.

Numerical Simulation of Making Hydrogen from Rich Filtration Combustion of Hydrogen Sulfide

Filtration combustion in porous media offers good advantages such as super-adiabatic combustion temperature in making hydrogen from hydrogen sulfur which is extremely toxic and is vastly produced in industry. In order to study the mechanism of making hydrogen from rich filtration combustion of hydrogen sulfur, computational fluid dynamics (CFD) were employed combining with a detail H2S oxidation mechanism to model the filtration combustion of hydrogen sulfide in a packed bed of uniform 3 mm diameter alumina spherical particles. The standard k-e turbulence model and a detail H2S oxidation mechanism with 17 species and 57 elemental reactions were adopted, and several equivalence ratios phi (1.0-4.0) were investigated. The numerical results agreed well with the experimental data, indicating that the combination of CFD with detail chemical kinetics gives good performance in modeling the anisotropic filtration flames. The combustion temperature exceeds the theoretical adiabatic combustion temperature by over 300 K, offering high temperature to decompose the hydrogen sulfide to hydrogen up to a conversion rate of 20%. On the other hand, the simulated combustion temperatures were relatively lower than the experimental data, resulting that much more un-burnt H2S (3.6% in simulation and 2.2% in experiment when phi=2.0) existed at the outlet. However, the predicted hydrogen concentrations were larger (3.3% in simulation and 2.0% in experimental when phi=2.0) than those measured in experiment.

Notice of Retraction Numerical simulation of combustion characteristics and pollutant emissions in a Stagnation Point Reverse Flow combustor

In order to study the mechanism of low NOX emissions and combustion characteristics in Stagnation Point Reverse Flow (SPRF) combustors, Computational Fluid Dynamics (CFD) method was employed to simulate the combustion process in a SPRF combustor. The distribution and variation of the major species, flow characteristics, pollutant emissions (CO and NO) were presented and discussed. Results showed that the reaction zone occupied approximate one third of the combustion chamber, resulting in much lower pollutant emissions. This implies that SPRF combustors work in flameless mode. Besides, the simplified two step methane oxidation mechanism combined with post-process technique to calculate NO emission has the capability to model the combustion process and pollutant emissions in SPRF combustors.