نشریه سوخت و احتراق
سال سوم شماره 1 (پیاپی 5، بهار و تابستان 1389)

This paper presents a research on thermal efficiency of home gas heaters. Installing baffles inside the furnace and against hot product gas flow prevents rapid gas flow out of the chimney, causing to increase the residue time of flue gases. Therefore, instead of heat energy loss, the energy is transferred to the room, thereby increasing the thermal efficiency of the gas heater. On the other hand, installing these baffles increases pressure drop, which can cause incomplete combustion and pollutant formation. In this work, we used fluent software to simulate fluid flow and heat transfer of product gases for different suggested obstacle surface configurations, and studied its effect on the thermal efficiency of gas heaters. Obstacle configuration with the maximum thermal efficiency is determined, built in workshop, installed and tested experimentally and its performance is shown to correspond with the computer simulation. Ultimately, the thermal efficiency of the model 12000 of Donar-khazar gas heater increased from 76.2 to 82.9% using this model.

The objective of this paper is to experimentally investigate the effects of Shchelkin spiral geometry in the transition from deflagration to detonation on the quality of the produced detonation. Deflagration to detonation transition (DDT) is required in cases where ignition energy release is less than the critical ignition energy required for a direct detonation initiation. A stoichiometric mixture of hydrogen–oxygen has been used in a tube of 25 mm diameter and 700 mm length. Spiral coils have been mounted at the ignition-end of the tube to facilitate DDT. Keeping spiral coil diameter fixed at 22mm, the coil length, pitch and thickness were varied to produce desirable detonation waves. Spiral coil selection was based on several criteria including; the pressure and velocity jump across the wave at the beginning of the tube, and the percentage of successful tests in each batch. The results indicate that the spiral coil pitch has the greatest influence on minimizing the DDT length and obtaining a desirable detonation.

In this work, an experimental investigation of stability limits of natural city gas (NG) in a non-premixed swirling burner has been conducted. Air swirl motion is imparted through 30º, 45º and 60º vane swirlers to produce low and high swirling air streams. Air dilution with nitrogen (N2) was also performed because of the importance of dilution in practical applications. N2 was added to the swirling air to examine the stability limits under diluted air conditions. The purpose of this study is to examine the stability limits including lift and blow-out velocity in low swirl conditions and liftoff height as well as liftoff velocity in high swirl conditions. It was observed that swirl increased the stability limits of NG while it decreased the negative effects of dilution on flame stability.

A methane-air turbulent premixed flame is simulated via probability density function (PDF) and Reynolds averaged Navier-Stokes (RANS) methods. In the PDF approach, molecular mixing is modelled through the modified Curl’s model. A Monte Carlo method is used to solve the PDF transport equation. Also, the run time averaging and local time stepping procedures are incorporated to increase the accuracy and reduce the computational time of the PDF simulation. In the RANS approach, the averaged chemical reaction rate term is modeled by the eddy breakup-finite rate model. A finite difference discretization on a staggered grid is utilized to obtain the numerical solution for the RANS equations. The characteristics and differences of the two above mentioned methods, including computational time and predicted mean fields, are investigated in detail for premixed flames. It is observed that the discrepancy of the predicted mean fields between the two methods is large especially in regions near the flame. In addition, the predicted flame length by the PDF method is approximately half the flame length predicted by the RANS method.

Dilution is one of the ways to reduce maximum temperature in the combustion chamber, which in turn leads to a reduction in the formation of thermal NO. The methods to control NOx formation are all based on temperature control and/or reduction in oxygen concentration. In the present paper, the numerical and experimental studies aim to examine the effect of N2 dilution on the formation of the pollutant NOx in the premixed flame of methane-air in the combustion chamber. The experimental results were obtained by designing a furnace with a cylindrical combustion chamber which was asymmetrical. The experiments were conducted with the equivalence ratios 0.7 to1.3 and various dilution ratios. Combustion simulation was studied using the Premix code of the CHEMKIN II version and various N2 dilution ratios within a range of equivalence ratios. The experimental and simulation results indicated that by increasing the ratio of dilution, maximum temperature of the flame and emission of the pollutant NOx from the combustion chamber decreases. The experimental and simulation results are also in good agreement and the trends of the obtained results conform with the experimental results of Knyazkov et al.