Few experiments have been performed to investigate the hydraulic performance in a chevron brazed plate heat exchanger (BPHE) with the narrow channel at lower Reynolds number. The hydraulic characteristics of seven types of chevron BPHEs were investigated experimentally and numerical simulation revealed the effects of structural parameters on hydraulic performances. The correlations between friction factor f and Re were fitted out based on more than 500 sets of pressure drop data. The research results show that there is a power-law between f and Re; which has a similar trend but a different amplitude for different plates, and the exponent of the power-law could be approximate to a constant. Numerical results show that the pressure drop Δp is positively correlated with the corrugated angle and spacing, however, negatively correlated with the corrugated height. Research on the hydraulic performance is significant for the optimal design of BPHE.

Experimental study on combustion instability of lean premixed swirl flame was conducted for methane-hydrogen mixtures, and the influence mechanism of hydrogen content (0%, 10%, 20%, and 40%) on the combustion instability under different acoustic frequencies (110–240 Hz) was investigated. A forced acoustic field created by a loud speaker was to simulate the complex and variable acoustic environment of unstable combustion. The flame transfer function was used to characterize the combustion instability. The morphologies of the flame, the evolution of vortex in the flame flow field, intensity of OH^* chemiluminescence of flame front structure, and the degree of local thermo-acoustic oscillation of flame were also investigated to facilitate this study. The results show that the increase in hydrogen content weakens the combustion instability of lean premixed swirl flame in the acoustic frequency ranges of 170–240 Hz, while the combustion instability is enhanced for the acoustic frequency ranges of 110–160 Hz. The main reason for this phenomenon is the increase in the hydrogen content changes development of the vortex in the flame flow field, the evolution of the flame surface and the intensity of OH^* chemiluminescence of flame front structure. This study provides an important reference for mastering the mechanism of the combustion instability of mixed gas flames and for developing technologies to inhibit combustion instability.

Lean premixed flames are useful for low nitrogen oxide (NO_x) emissions but more prone to induce combustion instability in gas turbines. Combustion instability of a lean premixed swirling flame (LPSF) with hydrogen–methane was investigated experimentally. The effects of hydrogen addition on combustion instability with equivalence ratios 0.75–1 were investigated with acoustic frequencies (90–240 Hz) and acoustic amplitudes (the ratio of velocity fluctuation to an average velocity of 0–0.5), respectively, which are characterized by the gain and phase of the flame describing function (FDF). The evolution of vortex and the flame morphologies were observed by the particle image velocimetry (PIV), intensified charge-coupled device (ICCD), photomultiplier tube (PMT), and Cassegrain optical systems. The global and local heat release fluctuations of the LPSF were shown by CH*/OH* chemiluminescence and temperature measurements. Results show that the FDF features maximum and minimum gain values in the acoustic frequency range of 90–240 Hz and reaches local maximum peaks at 110 and 180 Hz and local minimum peaks at 160 Hz. It can also be observed that varying velocity amplitudes (0–0.5) have greater effects on the gain and phase of FDF than changing equivalence ratios (0.75–1) for lean swirling flames. Higher velocity amplitudes more effectively intensified the compression of the flame length, which enhanced the mixing of the high-burning gas and the unburned gas, and then heat release fluctuations increased. However, it is more interesting that the effects of hydrogen addition on the combustion instability of the LPSF show a completely opposite phenomenon due to acoustic frequency under all experimental conditions. The FDFs were compared at typical frequencies of 140 and 180 Hz, and it was found that combustion instability enhanced with increasing hydrogen content at 140 Hz while weakened at 180 Hz. The flow field of PIV images shows that it is related to the location and development of vortices in the flame with varying acoustic frequencies. The intensity of OH*/CH* chemiluminescence, local temperature, and heat release rate show the same changing trend with the flame morphology for two acoustic parameters with the increasing hydrogen content in the LPSF. This directly affects the compression and curvature of the LPSF and thereby changes the mixture and temperature of the combustible gas, which influence the heat release fluctuation of the LPSF.

The effects of acoustic frequency (f)/0–400 Hz and amplitude (A)/0–1400 Pa on nitrogen oxides (NOx) emissions of a partially premixed flame were investigated experimentally. The mechanism of NOx emissions was analyzed by the evolution of the vortex, which was shown by particle image velocimetry (PIV). From the relationship of NOx emission index (EINOx) and acoustic parameters, it was concluded that a critical frequency (f_c) from 170 Hz to 190 Hz appeared. When the frequency was less than f_c, EINOx decreased linearly with an increase in amplitude. The flame length became shorter, which led to a decrease in the global residence time, and hence, a reduction in reaction time for NOx. However, a direct proportional relationship between EINOx and amplitude was not found when the frequency was larger than f_c. Based on PIV particle scattering images, with an increase of the acoustic frequency, the effects of the acoustic field on the flame base became less significant, but the flame length and reaction space of NOx were gradually increased.

The effects of plasma on the combustion instability of a methane swirling premixed flame under acoustic excitation were investigated. The flame image of OH planar laser-induced fluorescence and the fluctuation of flame transfer function showed the mechanism of plasma in combustion instability. The results show that when the acoustic frequency is less than 100 Hz, the gain in flame transfer function gradually increases with the frequency; when the acoustic frequency is 100~220 Hz, the flame transfer function shows a trend of first decreasing and then increasing with acoustic frequency. When the acoustic frequency is greater than 220 Hz, the flame transfer function gradually decreases with acoustic frequency. When the voltage exceeds the critical discharge value of 5.3 kV, the premixed gas is ionized and the heat release rate increases significantly, thereby reducing the gain in flame transfer function and enhancing flame stability. Plasma causes changes in the internal recirculation zone, compression, and curling degree of the flame, and thereby accelerates the rate of chemical reaction and leads to an increase in flame heat release rate. Eventually, the concentration of OH radicals changes, and the heat release rate changes accordingly, which ultimately changes the combustion instability of the swirling flame.