Variation Of Gas Temperature Research Articles

The analytical model of flame characteristics of hydrogen–air through wall and gas interaction analysis

AbstractIn this article, the effect of different boundary conditions and different thermal and physical properties of walls and gas on flame characteristics and stability of hydrogen–air mixture are investigated using an analytical method. This method solves the gas–wall energy equation, and the hydrogen mass conservation equations. The jump conditions are obtained by integrating the energy and mass equation into a small control volume around the flame. For validation of this model, the temperature distribution on the outer surface of the wall is compared with experimental data that show the maximum relative error of 3.5% for Q = 400 mL/min and 4.9% for Q = 200 mL/min. The maximum variation of gas temperature is nearly 6.5 times of wall temperature variation. The wall can be considered one‐dimensional for conventional wall materials with K > 10. For the existence of combustion inside the chamber, when the value of K is greater than 10, the Péclet number should also be considered greater than 10. In a constant equivalence ratio, increasing the medium temperature increases flame stability.

Non-invasive ultrasonic sensing of internal conditions on a partial full-scale spent nuclear fuel canister mock-up

The safe storage of spent nuclear fuel (SNF) in dry cask storage systems (DCSSs) is critical to the nuclear fuel cycle and the future of nuclear energy. A critical component of DCSSs is the SNF canister. The canister is a sealed stainless-steel structure, which is first vacuum dried and then backfilled with helium. The structural deterioration within a canister can be monitored through its internal gas properties. This monitoring serves as the driving force behind the non-invasive ultrasonic sensing approach in this paper. A major challenge in collecting gas-borne signals using ultrasonic sensing is the impedance mismatch between the stainless-steel canister and the helium gas inside. Only a small fraction of the ultrasonic signal makes its way from the transmitter to the receiver through the gas medium. In this paper, experimental studies on a partial full-scale canister mock-up were carried out to capture the gas-borne signals. Damping materials were applied on the outside, and blocking and unblocking tests were conducted to identify the gas-borne signal. The research results showed that the excitation frequency played an important role in maximizing the gas-borne signals. The gas-borne signal was successfully detected at around the theoretical time-of-flight (TOF) at 225 kHz. A high signal-to-noise ratio (SNR) was achieved in the measurements. Next, acoustic impedance matching (AIM) layers were added, and it was found that the gas signal energy was improved by 160.4% compared with that of no AIM layers. Subsequently, the relative humidity (RH) level and temperature of the gas were varied to simulate abnormal internal conditions of the canister. The non-invasive testing system demonstrated reliability and sensitivity in detecting gas temperature and RH variations. Theoretical calculations demonstrated the potential for detecting low-level xenon and air within an actual SNF canister filled with helium. Last, an active noise cancellation (ANC) method, previously developed by the authors, was verified on the canister mock-up for the first time. The results showed that the SNR of the gas signal was improved by 213.6% compared with that of no ANC.

Gas-solid flow behavior and heat transfer in a spiral-based reactor for calcium-based thermochemical energy storage

Calcium-based thermochemical energy storage presents a viable solution to bridge the gap between energy supply and demand during periods of off-sunlight due to its high energy density, low cost, and widespread availability. However, this technology faces a challenge in enhancing heat and mass transfer in the reactor, largely due to its low thermal conductivity. This study investigated the flow behavior and heat transfer in a spiral-based reactor, both with and without the inclusion of calcium-based particles during pneumatic conveying. The results indicate that, for flow behavior, the variation in Re number with a limited amount of particle mass in the spiral-based reactor is similar to that without particle conveying, increasing with an increase in gas velocity. However, an increased amount of particle mass can lead to clogging in the spiral-based reactor, resulting in notches in the Re number variation. Furthermore, the kinetic energy variation of calcium-based particles during pneumatic conveying follows four distinct stages: rapid growth, transient stationary, secondary growth, and secondary stationary, which increases with an increase in particle mass and gas velocity. For heat transfer, the temperature remains constant in the spiral-based reactor without particle conveying. However, the variation in gas temperature is opposite to that of particle temperature due to heat transfer between them in the spiral-based reactor during pneumatic conveying. The particle temperature increases with a reduction in particle size and feeding rate, and an increase in gas velocity. Additionally, the heat transfer rate increases with a reduction in particle size and an increase in gas velocity, but the particle feeding rate has a limited effect on heat transfer rate due to the same particle size. Finally, the research on effect of flow behavior on heat transfer suggests that superior heat transfer performance is more readily achieved under the gas-solid flow condition characterized by the lower kinetic energy and larger Re number.

Part-load analysis and preliminary annual simulation of a constant inventory supercritical CO2 power plant for waste heat recovery in cement industry

The present work investigates the part-load performance of a MW-scale sCO2 power plant designed as waste heat recovery unit for an existing cement plant located in Czech Republic, in the framework of the H2020 funded project CO2OLHEAT. The study first presents the selected power plant configuration and then focuses on the evaluation of its part-load operation due to variation of flue gas mass flow rate and temperature. The range of flue gas conditions at the outlet of the upstream process is retrieved from a preliminary statistical analysis of historical trends obtained through the cement plant monitoring. The numerical model developed for this study aims at providing realistic results thanks to the adoption of turbomachinery performance maps provided by the turbomachinery manufacturer of the project. Moreover, heat exchangers have been modelled through a discretized approach which has been validated against manufacturer data, while piping inventory and pressure losses have been assessed through a preliminary sizing that considers the actual distances to be covered in the cement plant. Performance decay is estimated for the whole range of flue gas conditions, reporting the most significant power cycle parameters, and identifying the main causes of efficiency loss. The part-load analysis is carried out considering a constant CO2 inventory, in order to reduce the system complexity and capital cost and simplify plant operation. Results show that the operation entails minor variation of the compressors operative points in the whole range of operating conditions of the cement plant, avoiding the risk of anti-surge bypass activation. Moreover, the plant is able to work close to the nominal thermodynamic cycle efficiency (20.5 %–23.0 %) for most of the year and benefits from part-load operation in terms of overall performance. In the last part of the work, a preliminary techno-economic analysis of the plant is also presented to highlight the potential advantages of sCO2 technology for waste heat recovery applications. The results of the part-load performance of the plant are combined with the flue gases data obtained from the preliminary statistical analysis and the cement plant historical monitoring. An annual electricity production equal to 13′909.7 MWh is obtained, corresponding to 6560 equivalent hours and a system capacity factor of 74.9 %. The investment cost of each CO2OLHEAT plant component is estimated by means of cost correlations obtained from literature and the non-discounted payback time is computed as a function of the electricity selling price. The results show that, even considering electricity prices before 2022, the payback time of the CO2OLHEAT plant is estimated to be lower than 8 years, justifying the industrial interest in the proposed technology.

Effects of spraying parameters and heat treatment temperature on microstructure and properties of single-pass and single-layer cold-sprayed Cu coatings on Al alloy substrate

Exploring the deposition of single-pass and single-layer pure Cu coatings on Al alloy substrate through high-pressure cold spray technology is a crucial endeavor with significant implications for advancing the utilization of such coatings. Regrettably, research in this specific area remains scarce, highlighting the need for further investigation and understanding. This study examined the characteristics of cold-sprayed Cu coatings deposited on a 6061 T6 aluminum alloy substrate under varying gas temperatures and pressures, and explored the effects of subsequent heat treatment on the microstructure and mechanical properties of the Cu coatings. The morphology, phase composition, surface roughness, thickness, porosity, microstructure, shear strength, and microhardness of the cold-sprayed Cu coatings were systematically analyzed. The findings indicated that the spraying parameters had a substantial impact on the properties of the Cu coatings. Specifically, increased gas pressure led to rougher surfaces and lower porosity, whereas higher gas temperature produced smoother surfaces and also reduced porosity. At 800 °C and 4.0 MPa, the Cu coatings exhibited the lowest surface roughness, measuring 8.03 μm. Conversely, at 800 °C and 5.5 MPa, the Cu coatings demonstrated the lowest porosity, at 0.26 %. Moreover, the microstructure analysis showed evidence of dynamic recrystallization in the deposited Cu particles, contributing to enhanced mechanical properties. Following this, the influence of heat treatment on the microstructure and properties of the cold-sprayed Cu coatings was examined. The results demonstrated that annealing at different temperatures induced changes in the interfacial microstructure, with the formation of intermetallic compounds between the Cu coating and Al alloy substrate. This interdiffusion phenomenon led to improved bonding strength and mechanical properties of the Cu coatings, as evidenced by increased shear strength. After undergoing heat treatment at 400 °C, the Cu coatings exhibited the highest shear strength, measuring 49.89 MPa. However, excessive heat treatment temperatures resulted in the formation of cracks and a decrease in mechanical properties.

Effect of knock on piston thermal load of a high compression ratio natural gas engine based on stepwise decoupling calculation

High compression ratio natural gas engine (NGE) is prone to knock at high load, which aggravates the thermal load of piston. Therefore, accurately evaluating the piston thermal load in the practical design is essential to enhance engine reliability and improve performance. The typical piston thermal load simulation adopts uniform wall boundary conditions, making it difficult to reveal the influence of turbulent flame and knock on the piston thermal load. In this paper, a stepwise decoupling method is applied to calculate the piston thermal load. The advancement of this method lies in its integration of chemical reaction mechanism with computational fluid dynamics (CFD) to calculate the spatial variations in heat transfer coefficient (HTC) and near-wall gas temperature at the piston top. Moreover, the conjugate heat transfer (CHT) method is adopted to solve the piston thermal load by coupling the thermal boundary conditions (TBC) obtained in the first step with the cooling oil boundary conditions. Based on this method, the effect of knock on piston thermal load of a high compression ratio NGE under different knock intensities is explored in this paper. It is found that turbulent combustion causes uneven distribution of thermal load at piston top. The decrease in knock intensity significantly reduces the gas temperature, heat flux and thermal load at piston top. Particularly under severe knock, the average temperature of piston is 22.9 K and 32.4 K higher than that of light knock and normal combustion, respectively. Furthermore, the main factor affecting the piston temperature field distribution is the gas temperature rather than the HTC. The gas temperature at piston top is overall the highest on the side near the inlet valve pit, which leads to a higher temperature on this side. Compared to light knock and normal combustion, when severe knock occurs, the thermal load on the exhaust valve pit increases significantly. This study provides an effective method for analyzing the heat transfer of the piston under knocking combustion, which is of guiding significance for controlling the thermal load of the high-temperature components in the combustion chamber.

Design of dual-channel Swiss-roll reactor for high-performance hydrogen production from ethanol steam reforming through waste heat valorization

Steam reforming is one of the most economical and efficient methods for hydrogen production. However, one of its glaring issues is the heat supply, which is used to maintain the reactions. Among reactors for hydrogen production, Swiss-roll reactors have exhibited exceptional performance in sustaining the heat required for reactions. Furthermore, heat supply via waste heat recovery has been an attractive industrial prospect for improving the sustainability of hydrogen production. This study proposes a design of a novel Swiss-roll reactor with dual channels to harvest waste heat from the flue gas to sustain the steam reforming reactions. Ethanol is used as a feedstock due to its environmentally benign properties. Numerical simulations are conducted to establish the reactor's catalytic reactions and heat transfer phenomena under the variations of gas hourly space velocity (GHSV), steam-to-ethanol (S/E) ratio, and inlet flue gas temperature. Heat recovery improves at higher GHSVs and lower S/E ratios at the cost of less efficient hydrogen production, while high inlet flue gas temperature improves heat recovery and hydrogen production. The novel reactor design can fulfill complete ethanol conversion maintained by heat recovered from waste flue gas, achieving 86.6 % hydrogen production efficiency and 72.7 % heat recovery under optimal operating conditions.

Degradation mechanism of measuring performance of optical particle counter under temperature-pressure coupling effect

To investigate the degradation mechanism of measuring the performance of an optical particle counter (OPC) under temperature-pressure coupling, this study first establishes a theoretical calculation model of gas refractive index and then elucidates the comprehensive influence mechanism of temperature and pressure on gas optical properties. Furthermore, the experimental measurement technique and measuring device for gas refractive index are built. By comparing the theoretical and experimental results in the temperature range of 48 °C–560 °C and the pressure range of 0.9–4.6 MPa, the difference between the two errors is just 0.05%, indicating the accuracy of the theoretical model of the refractive index of gas. Secondly, a dynamic model of optical measurement volume (OMV) under high-temperature and high-pressure conditions was established using geometrical optics theory, and the impacts of gas temperature and pressure variations on OPC measurement performance were investigated. The gas temperature (100 °C–1000 °C) and pressure (1–4.6 MPa) are shown to have opposing effects on the OMV, with gas pressure being more relevant. Finally, in order to eliminate the effect of gas refractive index change on the optical measurement performance of OPC, a parallel light model is proposed to solve the problem of the degradation of OPC measurement performance under temperature and pressure coupling conditions.

Determining Steady-State Operation Criteria Using Transient Performance Modelling and Steady-State Diagnostics

Data from the steady-state operation of gas turbine engines are used in gas path diagnostic procedures. A method to identify steady-state operation is thus required. This paper initially explains and demonstrates the factors that cause a deviation in engine health when transient data are used for diagnosis and shows that there is a threshold in the slope of time traces, below which the variation in engine health parameters is acceptable. A methodology for deriving a criterion for steady-state operation based on actual flight data is then presented. The slope of the exhaust gas temperature variation with time and the size of its time-series window, from which this slope is determined, are the required parameters that must be specified when applying this criterion. It is found that the values of these parameters must be selected so that a sufficient number of steady-state points are available without compromising the accuracy of the diagnostic procedure.

Variations in gas temperature and average velocity during laminar-turbulent transition in high-speed gas flow through microtubes

The primary objective of this study is to investigate variations in gas temperature and average velocity during the laminar-turbulent transition of compressible flow. This investigation encompasses an exploration of the effect of Mach number on transitional Reynolds number. Experiments were conducted utilizing a pair of adiabatic stainless steel microtubes with a diameter (D) of 131.6 μm. The first microtube was dedicated to measuring local pressure to obtain friction factor, local Mach number, and bulk temperature. The second microtube was employed for measuring local wall temperature. Additionally, a stainless steel microtube with a diameter (D) of 124 μm and a rectangular microchannel with a hydraulic diameter (Dh) of 99.36 μm were incorporated in the study. Friction factor, Mach number, and bulk temperature were determined by measuring mass flow rate and local pressure near the outlet. The measured mass flow rate exhibited a slight increase that plateaued during the laminar-turbulent transition as the Reynolds number increased. In laminar flow, an increase in the Reynolds number resulted in an increase of the Mach number and a reduction in bulk temperature. Conversely, during the laminar-turbulent transition, the Mach number either remained constant or decreased, and the bulk temperature increased due to the conversion of kinetic energy to thermal energy. Experimental validation of this phenomenon was achieved by measuring the wall temperature of an adiabatic microtube.

Influence of oxyhydrogen gas retrofit into two-stroke engine on emissions and exhaust gas temperature variations

The generation of power and fuel sustainability that contributes to a cleaner output of exhaust gases is one of the most important objectives the world seeks. In this paper, oxyhydrogen gas is used to retrofit into a two-stroke engine. The water was electrolysed and generated a mixture of oxygen (O2) and hydrogen (H2) or known as oxyhydrogen (HHO) gas via an electrolytic dry cell generator. The HHO was retrofitted experimentally to investigate the engine emissions and exhaust gas temperature from a 1.5 kW gasoline engine. The engine was tested with different power ratings (84–720 W) to investigate the performance and emissions of the engine using gasoline followed by the addition of HHO. The emissions of CO and NOx were measured with different amounts of HHO added. The exhaust temperature was calculated as one of the variables to be considered in relation to pollution. The air-fuel ratios are varied from 12 to 20% in the experiment. The most appropriate air-fuel ratio needed to start the generator with the most environmentally friendly gas emission was analysed. The results showed that the addition of HHO to the engine is successful in reducing fuel consumption up to 8.9%. A higher percentage of HHO added also has improved the emissions and reduced exhaust gas temperature. In this study, the highest quantity of HHO added at 0.15% of the volume fraction reduced CO gas emission by up to 9.41%, NOx gas up to 4.31%, and exhaust gas temperature by up to 2.02%. Generally, adding oxyhydrogen gas has significantly reduced the emissions, and exhaust temperature and provided an eco-friendly environment.

Effect of gas temperature on carbon soot oxidation via non-thermal plasma: two-dimensional numerical study integrating reactive flow and discharge models.

Non-thermal plasma (NTP) efficiently regenerates diesel particulate filters by oxidizing carbon soot (CS) at low temperatures. However, numerical studies on the spatial characteristics of CS oxidation by NTP are scarce. In addition, the influence of background gas heating on the CS-oxidizing performance by NTP remains inadequately understood. This research investigates the impact of gas temperature (323-573K) on heterogeneous CS oxidation using NTP in a two-dimensional configuration. The results indicate that CS is mainly oxidized by [Formula: see text], [Formula: see text], and [Formula: see text] during NTP treatment. The energy efficiency of CS removal by NTP ranges from 0.1 to 2.6g kWh-1 for varying gas temperature and applied voltage, consistent with previous research. Higher gas temperatures enhance both CS removal rate and efficiency, whereas higher applied voltages enhance rate at the expense of efficiency. The study also assesses energy conversion efficiency from electrical power input to chemical bonding energy during CS oxidation by NTP, yielding 0.03 to 0.23% efficiency for the considered gas temperature and voltage ranges, with higher temperatures leading to better efficiency.

Data fusion of spectral and acoustic signals in LIBS to improve the measurement accuracy of carbon emissions at varying gas temperatures

A novel mid-level data fusion method integrating spectral and acoustic signals of laser-induced plasmas was proposed to improve the measurement accuracy of carbon concentrations in flue gas at varying gas temperatures.

Comparative study on the flame retardancy of CO2 and N2 during coal adiabatic oxidation process

To test the effectiveness of N2 and CO2 in preventing coal from spontaneously combusting, researchers used an adiabatic oxidation apparatus to conduct an experiment with different temperature starting points. Non-adsorbed helium (He) was used as a reference gas, and coal and oxygen concentration temperature variations were analyzed after inerting. The results showed that He had the best cooling effect, N2 was second, and CO2 was the worst. At 70℃ and 110℃, the impact of different gases on reducing oxygen concentration and the cooling effect was the same. However, at the starting temperature of 150℃, CO2 was less effective in lowering oxygen concentration at the later stage than He and N2. N2 and CO2 can prolong the flame retardation time of inert gas and reduce oxygen displacement with an initial temperature increase. When the starting temperature is the same, N2 injection cools coal samples and replaces oxygen more effectively than CO2 injection. The flame retardancy of inert gas is the combined result of the cooling effect of inert gas and the replacement of oxygen. These findings are essential for using inert flame retardant technology in the goaf.

CFD simulation with analytical verification of discharging of nitrogen and helium from a high-pressure gas vessel

Many applications and industrial processes may necessitate the storage of large amounts of gases at high pressures. These compressed gases could be extremely dangerous if unintentionally released. since they could be poisonous or combustible when mixed with air. This research seeks to create a CFD model and undertake an analytical investigation of the process of de-pressurization of high-pressure gas from a vessel. The vessel to be studied is used in a pneumatic power system used to operate the control surface of a flying vehicle. The study involved two working fluids (nitrogen and helium). The discharging process is considered as a typical mass and heat transfer problem solved analytically by adopting The First Law of Thermodynamics to an unsteady flow control volume and simulated by employing transient flow analysis to the CFD model. The research was carried out on 440 bars (absolute pressure) of pressurized gas vessels. Working gases for the study included nitrogen and helium. Analytical solution results for the two different gases have been compared with the CFD simulation results to assess the CFD model’s accuracy. A good match is achieved when the gas temperature, pressure, density, and outlet mass flow rate variations are compared. The results demonstrated a good match between the CFD and analytical data, proving the validity of the CFD model.

Thermal–Fluid–Solid Coupling Analysis on the Effect of Cooling Gas Temperature on the Fatigue Life of Turbine Blades with TBCs

The application of thermal barrier coatings (TBCs) can increase the blade’s operating temperature and fatigue life. Previous studies have neglected the effects of cooling gas temperature variations on the temperature field, stress field, and fatigue life of blades and TBCs. For this reason, this paper considers the inhomogeneity of the high-temperature gas inlet temperature, the internal cooling gas temperature, the periodicity of the external flow field, etc., and establishes a finite element model of the gas turbine blade with TBCs. Then, the life of the blade and the TBCs is predicted based on the Ncode 2020R2. Finally, the effect of the cooling gas temperature on the temperature, stresses, and life of the blade and the TBCs is analyzed. The results show that the fatigue life of the TBCs is lower than that of the blade, and the low-life region of the TBCs is located at the leading edge of the blade, which is consistent with the TBCs shedding region of the real blade and verifies the accuracy of the life prediction method in this paper. The fatigue life of the blade and TBCs firstly increases and then decreases with the increase in the cooling gas temperature, and the trend of the stress changes in the opposite direction. When the cooling gas temperature is increased from 573 K to 873 K, the minimum life of the blade is increased by 358.5%, and that of the TBCs is increased by 138.7%. The conclusions can provide guidance for the design of long-life turbine blades with TBCs.

A Value-added COSMOS2020 Catalog of Physical Properties: Constraining Temperature-dependent Initial Mass Function

This work presents and releases a catalog of new photometrically derived physical properties for the ∼105 most well-measured galaxies in the COSMOS field on the sky. Using a recently developed technique, spectral energy distributions are modeled assuming a stellar initial mass function (IMF) that depends on the temperature of gas in star-forming regions. The method is applied to the largest current sample of high-quality panchromatic photometry, the COSMOS2020 catalog, that allows for testing this assumption. It is found that the galaxies exhibit a continuum of IMF and gas temperatures, most of which are bottom-lighter than measured in the Milky Way. As a consequence, the stellar masses and star formation rates of most galaxies here are found to be lower than those measured by traditional techniques in the COSMOS2020 catalog by factors of ∼1.6–3.5 and 2.5–70.0, respectively, with the change being the strongest for the most active galaxies. The resulting physical properties provide new insights into variation of the IMF-derived gas temperature along the star-forming main sequence and at quiescence, produce a sharp and coherent picture of downsizing, as seen from the stellar mass functions, and hint at a possible high-temperature and high-density stage of early galactic evolution.

Off-design and control performance analysis of a novel supercritical carbon dioxide power cycle for waste heat recovery

A novel supercritical carbon dioxide power cycle (sCO2) for waste heat recovery (WHR) is developed. This novel cycle has high efficiency, and it could make a more complete utilization of the waste heat. Besides, the flexible and independent controls of recompression compressor and main compressor can be made to obtain the better off-design performance. The off-design and control performance of this system is quantitatively investigated in this paper. The results indicate that the optimal net power output increases as mass flow rate or temperature of flue gas increases, whereas it decreases with increasing ambient temperature. The first and second highest exergy destruction shares always belong to the cooler and LTWHE under various external conditions. The adjustment of two compressor outlet pressures should be a priority under various mass flow rates or temperatures of flue gas, while the adjustment of cycle minimum pressure should be given a high-priority rating in response to variation of ambient temperature. Moreover, the control performance of recompression compressor is more sensitive to the variation of flue gas mass flow rate, while the control performance of main compressor is more sensitive to the variation of flue gas temperature and ambient temperature.

Feasibility investigation of non-equilibrium condensing nitrogen droplets as tracer particles in cryogenic wind tunnels

Application of high-precision non-intrusive flow field measurement techniques in transonic cryogenic wind tunnels (CWT) is confined to the lack of suitable tracer particles. The present study explored the feasibility of utilizing non-equilibrium condensing nitrogen droplets as tracer particles. A numerical model of the nitrogen non-equilibrium condensation was established. Accordingly, the non-equilibrium condensation process of nitrogen droplets and feasibility analysis of tracing were investigated. The results reveal that the occurrence of nitrogen droplets as well as the droplet concentration and mean radius can be regulated in the nucleation zone and droplet growth zone by varying gas temperature or pressure. Furthermore, the feasibility of employing nitrogen droplets as tracer particles in the working conditions of CWT is confirmed through the analysis of tracking capability, response time, concentration, and duration of droplet traceability. Formulas correlating used to calculate duration of droplet traceability have been proposed. This study proved the potential application of condensing nitrogen droplets as tracer particles in CWT.

The Physical Drivers and Observational Tracers of CO-to-H2 Conversion Factor Variations in Nearby Barred Galaxy Centers

The CO-to-H2 conversion factor (α CO) is central to measuring the amount and properties of molecular gas. It is known to vary with environmental conditions, and previous studies have revealed lower α CO in the centers of some barred galaxies on kiloparsec scales. To unveil the physical drivers of such variations, we obtained Atacama Large Millimeter/submillimeter Array bands (3), (6), and (7) observations toward the inner ∼2 kpc of NGC 3627 and NGC 4321 tracing 12CO, 13CO, and C18O lines on ∼100 pc scales. Our multiline modeling and Bayesian likelihood analysis of these data sets reveal variations of molecular gas density, temperature, optical depth, and velocity dispersion, which are among the key drivers of α CO. The central 300 pc nuclei in both galaxies show strong enhancement of temperature T k ≳ 100 K and density cm−3. Assuming a CO-to-H2 abundance of 3 × 10−4, we derive 4–15 times lower α CO than the Galactic value across our maps, which agrees well with previous kiloparsec-scale measurements. Combining the results with our previous work on NGC 3351, we find a strong correlation of α CO with low-J 12CO optical depths (τ CO), as well as an anticorrelation with T k. The τ CO correlation explains most of the α CO variation in the three galaxy centers, whereas changes in T k influence α CO to second order. Overall, the observed line width and 12CO/13CO 2–1 line ratio correlate with τ CO variation in these centers, and thus they are useful observational indicators for α CO variation. We also test current simulation-based α CO prescriptions and find a systematic overprediction, which likely originates from the mismatch of gas conditions between our data and the simulations.