Phys Sci Res Assoc, Mechanical Engineering
Doctor of Philosophy, Stanford University, EE-PMN (2017)
Doctor of Philosophy, Stanford University, ME-PHD (2017)
Master of Science, Stanford University, ME-MS (2012)
Bachelor of Science, Peking University, Theoretical& Applied Mechanics (2010)
Quantitative 2-D OH thermometry using spectrally resolved planar laser-induced fluorescence
2019; 44 (3): 578–81
A novel method is presented for quantitative two-dimensional temperature measurement in combustion gases. This method, namely spectrally resolved planar laser-induced fluorescence thermometry, utilizes a high-power, wavelength-tunable and narrow-linewidth CW laser to access the spectral lineshapes of a key combustion intermediate, the hydroxyl radical (OH), and enables high-fidelity and calibration-free quantification of non-uniform temperature fields in complex reacting flows. Specifically, the R1(11)/R1(7) line pair of the OH A2Σ+-X2Π(0,0) rovibronic band was probed with laser radiation near 306.5 nm, and their fluorescence ratios were used to infer temperature. Preliminary demonstrations of this thermometry method were performed in a series of burner-stabilized CH4-air flames.
View details for DOI 10.1364/OL.44.000578
View details for Web of Science ID 000457292400029
View details for PubMedID 30702683
- Shock tube measurements of OH concentration time-histories in benzene, toluene, ethylbenzene and xylene oxidation PROCEEDINGS OF THE COMBUSTION INSTITUTE 2019; 37 (1): 163–70
- Demonstration of non-absorbing interference rejection using wavelength modulation spectroscopy in high-pressure shock tubes APPLIED PHYSICS B-LASERS AND OPTICS 2019; 125 (1)
- Cavity-enhanced absorption spectroscopy for shocktubes: Design and optimization PROCEEDINGS OF THE COMBUSTION INSTITUTE 2019; 37 (2): 1345–53
- Shock tube study of normal heptane first-stage ignition near 3.5 atm COMBUSTION AND FLAME 2018; 198: 376–92
- A shock tube study of jet fuel pyrolysis and ignition at elevated pressures and temperatures FUEL 2018; 226: 338–44
Ultra-sensitive spectroscopy of OH radical in high-temperature transient reactions
2018; 43 (15): 3518–21
The hydroxyl (OH) radical is arguably the most important transient radical in high-temperature gas-phase combustion reactions, yet it is very difficult to measure because of its high reactivity and, thus, short lifetime and low concentration. This work reports the development of a novel method for ultra-sensitive, quantitative, and microsecond-resolved detection of OH based on UV frequency-modulation spectroscopy (FMS). To the best of the authors' knowledge, this is the first FMS demonstration in the near-UV spectral region for detection of short-lived radical species. Shot-noise-limited detection was achieved at an optical power of 25 mW. A proof-of-concept experiment in a tabletop H2O/He microwave discharge cell has reached a 1σ minimum detectable absorbance (MDA) of less than 2×10-4 over 1 MHz measurement bandwidth. High-temperature OH measurement was demonstrated in a 15 cm diameter shock tube, where a typical MDA of 3.0×10-4 was achieved at 1330 K, 0.38 atm, and 1 MHz. These preliminary results have outperformed the previous best MDA by more than a factor of 3; further improvement by another order of magnitude is anticipated, following the strategies outlined at the end of this Letter. The current method paves the path to parts per billion (ppb) -level OH detection capability and offers prospects to significantly advance fundamental combustion research by enabling direct observation of OH formation and scavenging kinetics during key stages of fuel oxidation that were inaccessible with previous methods.
View details for DOI 10.1364/OL.43.003518
View details for Web of Science ID 000440405900016
View details for PubMedID 30067624
- A physics-based approach to modeling real-fuel combustion chemistry - II. Reaction kinetic models of jet and rocket fuels COMBUSTION AND FLAME 2018; 193: 520–37
- High-sensitivity 308.6-nm laser absorption diagnostic optimized for OH measurement in shock tube combustion studies APPLIED PHYSICS B-LASERS AND OPTICS 2018; 124 (3)
- A new diagnostic for hydrocarbon fuels using 3.41-mu m diode laser absorption COMBUSTION AND FLAME 2017; 186: 129–39
- Time-resolved sub-ppm CH3 detection in a shock tube using cavity-enhanced absorption spectroscopy with a ps-pulsed UV laser PROCEEDINGS OF THE COMBUSTION INSTITUTE 2017; 36 (3): 4549-4556
Shock Tube and Laser Absorption Study of CH2O Oxidation via Simultaneous Measurements of OH and CO.
The journal of physical chemistry. A
2017; 121 (45): 8561–68
The oxidation of Ar-diluted stoichiometric CH2O-O2 mixtures was studied behind reflected shock waves over temperatures of 1332-1685 K, at pressures of about 1.5 atm and initial CH2O mole fractions of 500, 1500, and 5000 ppm. Quantitative and time-resolved concentration histories of OH and CO (at both v″ = 0 and v″ = 1) were measured by narrow-linewidth laser absorption at 306.7 and 4854 nm, respectively. A time delay was observed between the formation of v″ = 0 and v″ = 1 states of CO, suggesting that CO was kinetically generated primarily in the ground state and then collisionally relaxed toward vibrational equilibrium. The measured CO and OH time-histories were used to evaluate the performance of four detailed reaction mechanisms regarding the oxidation chemistry of CH2O. Further analyses of these time-history data have also led to improved determination for the rate constants of two key reactions, namely H + O2 = O + OH (R1) and OH + CO = CO2 + H (R2), as follows: k1 = 8.04 × 1013 exp(-7370 K/T) cm3 mol-1 s-1, k2 = 1.90 × 1012 exp(-2760 K/T) cm3 mol-1 s-1; both expressions are valid over 1428-1685 K and have 1σ uncertainties of approximately ±10%.
View details for PubMedID 29065683
- Rate constants of long, branched, and unsaturated aldehydes with OH at elevated temperatures PROCEEDINGS OF THE COMBUSTION INSTITUTE 2017; 36 (1): 151-160
- Kinetics of Excited Oxygen Formation in Shock-Heated O-2-Ar Mixtures JOURNAL OF PHYSICAL CHEMISTRY A 2016; 120 (42): 8234-8243
Shock Tube Measurement for the Dissociation Rate Constant of Acetaldehyde Using Sensitive CO Diagnostics.
journal of physical chemistry. A
2016; 120 (35): 6895-6901
The rate constant of acetaldehyde thermal dissociation, CH3CHO = CH3 + HCO, was measured behind reflected shock waves at temperatures of 1273-1618 K and pressures near 1.6 and 0.34 atm. The current measurement utilized sensitive CO diagnostics to track the dissociation of CH3CHO via oxygen atom balance and inferred the title rate constant (k1) from CO time histories obtained in pyrolysis experiments of 1000 and 50 ppm of CH3CHO/Ar mixtures. By using dilute test mixtures, the current study successfully suppressed the interferences from secondary reactions and directly determined the title rate constant as k1(1.6 atm) = 1.1 × 10(14) exp(-36 700 K/T) s(-1) over 1273-1618 K and k1(0.34 atm) = 5.5 × 10(12) exp(-32 900 K/T) s(-1) over 1377-1571 K, with 2σ uncertainties of approximately ±30% for both expressions. Example simulations of existing reaction mechanisms updated with the current values of k1 demonstrated substantial improvements with regards to the acetaldehyde pyrolysis chemistry.
View details for DOI 10.1021/acs.jpca.6b03647
View details for PubMedID 27523494
Improved Shock Tube Measurement of the CH4 + Ar = CH3 + H + Ar Rate Constant using UV Cavity-Enhanced Absorption Spectroscopy of CH3.
journal of physical chemistry. A
2016; 120 (28): 5427-5434
We report an improved measurement for the rate constant of methane dissociation in argon (CH4 + Ar = CH3 + H + Ar) behind reflected shock waves. The experiment was conducted using a sub-parts per million sensitivity CH3 diagnostic recently developed in our laboratory based on ultraviolet cavity-enhanced absorption spectroscopy. The high sensitivity of this diagnostic allowed for measurements of quantitatively resolved CH3 time histories during the initial stage of CH4 pyrolysis, where the reaction system is clean and free from influences of secondary reactions and temperature change. This high sensitivity also allowed extension of our measurement range to much lower temperatures (<1500 K). The current-reflected shock measurements were performed at temperatures between 1487 and 1866 K and pressures near 1.7 atm, resulting in the following Arrhenius rate constant expression for the title reaction: k(1.7 atm) = 3.7 × 10(16) exp(-42 200 K/T) cm(3)/mol·s, with a 2σ uncertainty factor of 1.25. The current data are in good consensus with various theoretical and review studies, but at the low temperature end they suggest a slightly higher (up to 35%) rate constant compared to these previous results. A re-evaluation of previous and current experimental data in the falloff region was also performed, yielding updated expressions for both the low-pressure limit and the high-pressure limit rate constants and improved agreement with all existing data.
View details for DOI 10.1021/acs.jpca.6b02572
View details for PubMedID 27380878
- Cavity-enhanced absorption spectroscopy with a ps-pulsed UV laser for sensitive, high-speed measurements in a shock tube OPTICS EXPRESS 2016; 24 (1): 308-318
Shock-tube measurements of excited oxygen atoms using cavity-enhanced absorption spectroscopy
2015; 54 (29): 8766-8775
We report the use of cavity-enhanced absorption spectroscopy (CEAS) using two distributed feedback diode lasers near 777.2 and 844.6 nm for sensitive, time-resolved, in situ measurements of excited-state populations of atomic oxygen in a shock tube. Here, a 1% O2/Ar mixture was shock-heated to 5400-8000 K behind reflected shock waves. The combined use of a low-finesse cavity, fast wavelength scanning of the lasers, and an off-axis alignment enabled measurements with 10 μs time response and low cavity noise. The CEAS absorption gain factors of 104 and 142 for the P35←S520 (777.2 nm) and P0,1,23←S310 (844.6 nm) atomic oxygen transitions, respectively, significantly improved the detection sensitivity over conventional single-pass measurements. This work demonstrates the potential of using CEAS to improve shock-tube studies of nonequilibrium electronic-excitation processes at high temperatures.
View details for DOI 10.1364/AO.54.008766
View details for Web of Science ID 000362667200028
View details for PubMedID 26479817
- Shock Tube Measurement of the High-Temperature Rate Constant for OH + CH3 -> Products JOURNAL OF PHYSICAL CHEMISTRY A 2015; 119 (33): 8799-8805
Shock-Tube Measurement of Acetone Dissociation Using Cavity-Enhanced Absorption Spectroscopy of CO.
journal of physical chemistry. A
2015; 119 (28): 7257-7262
A direct measurement for the rate constant of the acetone dissociation reaction (CH3COCH3 = CH3CO + CH3) was conducted behind reflected shock wave, utilizing a sub-ppm sensitivity CO diagnostic achieved by cavity-enhanced absorption spectroscopy (CEAS). The current experiment eliminated the influence from secondary reactions and temperature change by investigating the clean pyrolysis of <20 ppm acetone in argon. For the first time, the acetone dissociation rate constant (k1) was directly measured over 5.5 orders of magnitude with a high degree of accuracy: k1 (1004-1494 K, 1.6 atm) = 4.39 × 10(55) T(-11.394) exp(-52 140K/T) ± 24% s(-1). This result was seen to agree with most previous studies and has bridged the gap between their temperature and pressure conditions. The current work also served as an example demonstration of the potential of using the CEAS technique in shock-tube kinetics studies.
View details for DOI 10.1021/jp511642a
View details for PubMedID 25659401
- High-sensitivity interference-free diagnostic for measurement of methane in shock tubes JOURNAL OF QUANTITATIVE SPECTROSCOPY & RADIATIVE TRANSFER 2015; 156: 80-87
- High temperature measurements for the rate constants of C-1-C-4 aldehydes with OH in a shock tube PROCEEDINGS OF THE COMBUSTION INSTITUTE 2015; 35: 473-480
Shock Tube Measurement of the High-Temperature Rate Constant for OH + CH3 → Products.
The journal of physical chemistry. A
2015; 119 (33): 8799–8805
The reaction between hydroxyl (OH) and methyl radicals (CH3) is critical to hydrocarbon oxidation. Motivated by the sparseness of its high-temperature rate constant data and the large uncertainties in the existing literature values, the current study has remeasured the overall rate constant of the OH + CH3 reaction and extended the measurement temperature range to 1214-1933 K, using simultaneous laser absorption diagnostics for OH and CH3 radicals behind incident and reflected shock waves. tert-Butyl hydroperoxide and azomethane were used as pyrolytic sources for the OH and CH3 radicals, respectively. The current study bridged the temperature ranges of existing experimental data, and good agreement is seen between the current measurement and some previous experimental and theoretical high-temperature studies. A recommendation for the rate constant expression of the title reaction, based on the weighted average of the high-temperature data from selected studies, is given by k1 = 4.19 × 10(1)(T/K)(3.15) exp(5270 K/T) cm(3) mol(-1) s(-1) ±30%, which is valid over 1000-2500 K.
View details for PubMedID 26230910
- Constrained reaction volume shock tube study of n-heptane oxidation: Ignition delay times and time-histories of multiple species and temperature PROCEEDINGS OF THE COMBUSTION INSTITUTE 2015; 35: 231-239
- Reaction Rate Constant of CH2O + H = HCO + H-2 Revisited: A Combined Study of Direct Shock Tube Measurement and Transition State Theory Calculation JOURNAL OF PHYSICAL CHEMISTRY A 2014; 118 (44): 10201-10209
- Effects of size polydispersity on electron mobility in a two-dimensional quantum-dot superlattice PHYSICAL REVIEW B 2014; 90 (14)
- Time-resolved in situ detection of CO in a shock tube using cavity-enhanced absorption spectroscopy with a quantum-cascade laser near 4.6 mu m OPTICS EXPRESS 2014; 22 (20): 24559-24565
Sensitive and rapid laser diagnostic for shock tube kinetics studies using cavity-enhanced absorption spectroscopy
2014; 22 (8): 9291-9300
We report the first application of cavity-enhanced absorption spectroscopy (CEAS) using a coherent light source for sensitive and rapid gaseous species time-history measurements in a shock tube. Off-axis alignment and fast scanning of the laser wavelength were used to minimize coupling noise in a low-finesse cavity. An absorption gain factor of 83 with a measurement time resolution of 20 µs was demonstrated for C2H2 detection using a near-infrared transition near 1537 nm, corresponding to a noise-equivalent detection limit of 20 ppm at 296 K and 76 ppm at 906 K at 50 kHz. This substantial gain in signal, relative to conventional single-pass absorption, will enable ultra-sensitive species detection in shock tube kinetics studies, particularly useful for measurements of minor species and for studies of dilute reactive systems.
View details for DOI 10.1364/OE.22.009291
View details for Web of Science ID 000335902200046
View details for PubMedID 24787817
Reaction rate constant of CH2O + H = HCO + H2 revisited: a combined study of direct shock tube measurement and transition state theory calculation.
The journal of physical chemistry. A
2014; 118 (44): 10201–9
The rate constant of the H-abstraction reaction of formaldehyde (CH2O) by hydrogen atoms (H), CH2O + H = H2 + HCO, has been studied behind reflected shock waves with use of a sensitive mid-IR laser absorption diagnostic for CO, over temperatures of 1304-2006 K and at pressures near 1 atm. C2H5I was used as an H atom precursor and 1,3,5-trioxane as the CH2O precursor, to generate a well-controlled CH2O/H reacting system. By designing the experiments to maintain relatively constant H atom concentrations, the current study significantly boosted the measurement sensitivity of the target reaction and suppressed the influence of interfering reactions. The measured CH2O + H rate constant can be expressed in modified Arrhenius from as kCH2O+H(1304-2006 K, 1 atm) = 1.97 × 10(11)(T/K)(1.06) exp(-3818 K/T) cm(3) mol(-1)s(-1), with uncertainty limits estimated to be +18%/-26%. A transition-state-theory (TST) calculation, using the CCSD(T)-F12/VTZ-F12 level of theory, is in good agreement with the shock tube measurement and extended the temperature range of the current study to 200-3000 K, over which a modified Arrhenius fit of the rate constant can be expressed as kCH2O+H(200-3000 K) = 5.86 × 10(3)(T/K)(3.13) exp(-762 K/T) cm(3) mol(-1)s(-1).
View details for PubMedID 25319141
- High-temperature laser absorption diagnostics for CH2O and CH3CHO and their application to shock tube kinetic studies COMBUSTION AND FLAME 2013; 160 (10): 1930-1938
- Constrained reaction volume approach for studying chemical kinetics behind reflected shock waves COMBUSTION AND FLAME 2013; 160 (9): 1550-1558
- On the rate constants of OH + HO2 and HO2 + HO2: A comprehensive study of H2O2 thermal decomposition using multi-species laser absorption PROCEEDINGS OF THE COMBUSTION INSTITUTE 2013; 34: 565-571