Wall Heat Loss Research Articles

Liquid ammonia spray combustion in two-stage micro gas turbine combustors at 0.25 MPa; Relevance of combustion enhancement to flame stability and NOx control

The development of liquid ammonia spray combustion technology is expected to reduce the cost and size of gas turbines otherwise fueled with gaseous ammonia. However, successful studies on this topic are scarce due to the difficulty in flame stabilization. In this study, the flame stability and emissions control in liquid ammonia spray combustion is studied for the first time for input thermal power up to 230 kW using novel two-stage gas turbine combustors with and without slot film cooling at 0.25 MPa. The experiments were conducted in a micro gas turbine combustor test facility originally developed for gaseous ammonia combustion. The results show that the injection of liquid ammonia spray into a methane-air flame encouraged rapid cooling of the combustor liner head, enhancing flame blowoff. The blowoff limit based on ammonia fraction in the fuel showed a strong dependence on the combustor temperature, hence the combustor without slot film cooling extended the blowoff limit allowing the successful stabilization of pure liquid ammonia spray flame. Result of emissions analysis showed that two-stage rich-lean combustion is effective in controlling emissions from the flames, however, requires further enhancement of the flame in the primary combustion zone. An increase in combustor inlet temperature and a decrease in combustor wall heat loss, both of which enhanced the combustion of the liquid spray, resulted in a decrease in the emissions of NO, NO2, N2O, NH3, HCN and CO from liquid ammonia co-fired with methane. Conversely, pure liquid ammonia spray combustion resulted in higher emissions of NH3, NO and N2O than did the co-combustion with methane, encouraged by inhomogeneity in droplets distribution and larger heat transfer to the liquid droplets.

Numerical comparison of premixed H2/air combustion characteristic of three types of micro cavity-combustors with guide vanes, bluff body, guide vanes and bluff body respectively

Micro-scale combustion is facing the problems of ignition difficulty, combustion instability, and low combustion efficiency. Therefore, it is necessary to improve the combustion characteristics in micro-combustor to expand the application range of micro-combustor. Focusing on the problem of the weak preheating effect of the micro cavity-combustor, the guide vanes are constructed to enhance the combustion near the cavity, and further combine with the bluff body to enhance combustion. The combustion characteristics of three types of cavity combustors with guide vanes (CCGV), bluff body (CCBB), bluff body, and guide vanes (CCGB) respectively are compared and analyzed under different inlet velocities (8–32 m/s), equivalence ratios (0.6–1.4), and wall materials (quartz glass, steel, and SiC). Results show that the guide vanes can greatly improve the combustion intensity near the cavity and improve the combustion stability of the cavity combustor. The combustion efficiency of CCGV and CCGB are increased by 43.04% and 85.96% respectively than CCBB when the inlet velocity is 32 m/s. The reaction heat of CCGV is 244.5 W when inlet velocity is 32 m/s, which is 0.55 times and 1.57 times that of CCGV and CCBB, respectively. The temperature uniformity and mean temperature of the outer wall of CCGV and CCGB both are better than that of CCGB. The combustion efficiency of CCGB is the highest among three combustors under the same equivalence ratio, especially when the equivalence ratio is less than 1. The reaction intensity in the cavity of the CCGV and CCGB with steel wall material is highest than that of the combustor with the other two wall materials. Wall materials with high thermal conductivity have a better preheating effect. Compared with quartz as the wall material, the mean temperature of the external wall of CCGV and CCGB using steel and silicon carbide as the wall material both increase by more than 130 K, and the wall heat loss both increase by more than 50%.

Computational Modeling of Boundary Layer Flashback in a Swirling Stratified Flame Using a LES-Based Non-Adiabatic Tabulated Chemistry Approach.

When operating under lean fuel–air conditions, flame flashback is an operational safety issue in stationary gas turbines. In particular, with the increased use of hydrogen, the propagation of the flame through the boundary layers into the mixing section becomes feasible. Typically, these mixing regions are not designed to hold a high-temperature flame and can lead to catastrophic failure of the gas turbine. Flame flashback along the boundary layers is a competition between chemical reactions in a turbulent flow, where fuel and air are incompletely mixed, and heat loss to the wall that promotes flame quenching. The focus of this work is to develop a comprehensive simulation approach to model boundary layer flashback, accounting for fuel–air stratification and wall heat loss. A large eddy simulation (LES) based framework is used, along with a tabulation-based combustion model. Different approaches to tabulation and the effect of wall heat loss are studied. An experimental flashback configuration is used to understand the predictive accuracy of the models. It is shown that diffusion-flame-based tabulation methods are better suited due to the flashback occurring in relatively low-strain and lean fuel–air mixtures. Further, the flashback is promoted by the formation of features such as flame tongues, which induce negative velocity separated boundary layer flow that promotes upstream flame motion. The wall heat loss alters the strength of these separated flows, which in turn affects the flashback propensity. Comparisons with experimental data for both non-reacting cases that quantify fuel–air mixing and reacting flashback cases are used to demonstrate predictive accuracy.

Large eddy simulation on flame topologies and the blow-off characteristics of ammonia/air flame in a model gas turbine combustor

Ammonia (NH3) is recently identified as one of the suitable energy carriers in hydrogen energy system. However, regarding NH3 as a fuel is still facing several challenges which limit the direct use on modern combustion systems. The main challenge is the difficulty to stabilize NH3/air flames. To reveal and analyze the stabilization mechanism and the characteristics during blow-off processes in a swirl combustor, large eddy simulation with thickened flame model was performed to resolve the three dimensional swirl NH3/air flame. The CH4/air flame was also performed for comparison. The reacting flow fields and the flame structures were measured by PIV and OH-PLIF technique respectively. For NH3/air flame, flame fronts are burning to the higher equivalence ratio region, which leads to a larger trend of extinguishing. The curvature distribution at flame root is mainly dependent on the flame characteristics. Downstream the flows, the curvature distribution is mainly influenced by the flow field characteristics. Though the value of wall heat loss of NH3/air flame is less than that of CH4/air flame, the heat loss still shows greater influence on NH3/air flame stabilization when considering the lower heat release rate of NH3/air flame. The blow-off of CH4/air flame is mainly caused by the decreasing HRR and heat loss, while for NH3/air flame, it is a combined effect of the excessive stretch, the reduction of HRR as well as the greater heat loss effect.

Direct Numerical Simulation of hydrogen combustion at auto-ignitive conditions: Ignition, stability and turbulent reaction-front velocity

Direct Numerical Simulations (DNS) are performed to investigate the process of spontaneous ignition of hydrogen flames at laminar, turbulent, adiabatic and non-adiabatic conditions. Mixtures of hydrogen and vitiated air at temperatures representing gas-turbine reheat combustion are considered. Adiabatic spontaneous ignition processes are investigated first, providing a quantitative characterization of stable and unstable flames. Results indicate that, in hydrogen reheat combustion, compressibility effects play a key role in flame stability and that unstable ignition and combustion are consistently encountered for reactant temperatures close to the mixture’s characteristic crossover temperature. Furthermore, it is also found that the characterization of the adiabatic processes is also valid in the presence of non-adiabaticity due to wall heat-loss. Finally, a quantitative characterization of the instantaneous fuel consumption rate within the reaction front is obtained and of its ability, at auto-ignitive conditions, to advance against the approaching turbulent flow of the reactants, for a range of different turbulence intensities, temperatures and pressure levels.

Effects of influencing factors on premixed CH4-O2 combustion in a cylindrical porous media combustor

Numerical studies have been conducted on pre-mixed methane-oxygen in a cylindrical porous-media combustor. The Central Composite Design model and the Response Surface Method based on the Genetic Aggregation Algorithm have been applied to investigate the synchronous behavior of key flow parameters on flame temperature, flame velocity, outlet temperature, outlet velocity and wall heat loss. The results show that the flame temperature, flame velocity and the heat loss from the wall increase when the operating parameters increase. Increasing the length of the combustor reduces the outlet temperature and velocity. Comparatively, the inlet velocity and porosity play the most significant roles among the operating parameters. Out of the various design modes, the maximum possible temperature and velocity of the flame are 2542.1 K and 17.252 m/s respectively and that at the combustor outlet are 1454.8 K and 11.129 m/s respectively. The tolerable maximum heat loss through the wall is 476.51 K.

Wall heat loss recapture evaluation of transpired solar collectors for different climates: A European case study

In the present paper, the performance of the transpired solar collector (TSC) is evaluated for several European cities with different climates. Two primary outputs, the solar air heating, and the recapture of transmission heat loss through the walls were analysed. The wall heat loss recapture can be considered as an important factor for energy saving in buildings, which has not been studied before. RETScreen and SWift are two building simulation tools available for the design and evaluation of transpired solar collectors. Another primary aim is to investigate the detailed mathematical approach that describes the heat transfer process of the physical model for each simulation tool. Results of the models were analysed to predict the performance of TSC in delivering heated air for ventilation applications, and the ability of the partial recapture the transmission heat loss from the warm building interior. Solar radiation, heating degree days, wall thermal transmittance, and air flowrate are the key parameters that influence the heat transfer process. The findings indicate a promising result of using TSC as a dynamic insulation solution for energy savings which can reach up to 55 [kWh/(m2·year)] at high air flowrate 180 [m3/(h∙m2)] for some climates besides the main duty which is the solar air heating.

Characteristics of wall heat transfer from impinging diesel spray flame in low oxygen concentration ambient

The objective of this study is to obtain an enhanced understanding of the effect of diesel spray/flame impingement on transient heat flux to the wall. By using a constant volume vessel under engine-like condition, surface heat flux of the wall at spray/flame impingement was measured with three thin film thermocouple heat flux sensors. Fuel was injected using a single-hole injector with a 0.133 mm diameter nozzle. In order to investigate the relation between diesel flame and wall heat loss, two color-method was applied to observe flame temperature distribution. 21 % and 16 % as the various oxygen concentration were investigated at each injection pressure of pressure 80, 120, and 180 MPa. The results showed that the lower oxygen concentration was generated heat loss regardless of injection pressures. The lower flame temperature and flame contact area and period were influence factor on heat loss on the wall. Furthermore, the relation between Nusselt number (Nu) and Reynolds number (Re) were also investigated to understand the heat transfer phenomena in diesel engines.

Mathematical Model of Maximum Commutation Half Cycle for Thermal Countercurrent Oxidation of Low‐Concentration Gas in Coal Mine Ventilation

The Fluent computational fluid dynamics software was used to study the relevant factors affecting the maximum commutation half cycle for thermal countercurrent oxidation of low‐concentration gas in coal mine ventilation. Based on orthogonal experiments, the maximum commutation half cycle for thermal countercurrent oxidation of the exhaust gas in the coal mine ventilation under 25 working conditions with the combination of different methane concentrations, inlet speeds, porosities, and oxidation bed filling lengths is investigated. SPSS data processing software was used to perform regression analysis on the numerical simulation data, and a mathematical model for predicting the maximum commutation half cycle under the influence of four factors was obtained. Through experiments, the mathematical model of the maximum commutation half cycle by the numerical simulation was verified. After introducing the wall heat loss correction coefficient, the complete prediction model of the maximum commutation half cycle was obtained. Comparing the experimental test value with the calculated value using the corrected model, the relative error was not more than 3%. The complete mathematical model corrected can be applied to the design calculation of the maximum commutation half cycle for thermal countercurrent oxidation of low‐concentration gas in actual coal mine ventilation.

Study on Safety Performance of Building Finish Layer under Thermomechanical Coupling Condition

The building finish layer is a comprehensive structural system including the building exterior insulation system and building exterior finish. Combining with buildings has the advantage of reducing wall heat loss and building deformation caused by large temperature differences. Since the building finish layer is prone to cracking, hollowing, and peeling, during the application process, its safety needs to be studied and certified. This study prepares 20 groups of specimens, 15 anchor bolts in each group. The anchor bolt pull‐out strength test is carried out. Anchoring damage evolution law and failure mode of anchor bolts are investigated. And the influence of anchoring methods on the pull‐out bearing capacity is analyzed. In addition, ABAQUS finite element data simulation is carried out. The stress state of finish in thermomechanical coupling condition and without the effect of temperature are compared and analyzed. The influence factors of anchor bolt pull‐out strength and the influence of temperature load on the long‐term performance of building finish layer are obtained. The durability of the building finish layer is analyzed. The results show that the anchoring strength of the anchor bolt is positively correlated with the anchoring depth. The anchoring strength is influenced significantly by anchoring construction sequence and temperature. The stress under the coupled effect of temperature and load is greater than that of the single effect of load, and the stress distribution changes significantly. Due to thermal expansion and contraction, the anchor bolt would loosen, which is more prone to damage the building finish layer in a low temperature environment. The weight relationship of each influencing factor of the building finish layer is proposed. A systematic evaluation index system is established. The results of this study provide a basis for subsequent related research work and engineering applications.

Effects of Isothermal Wall Boundary Conditions on Rotating Detonation Engine

ABSTRACT Three-dimensional numerical simulations of rotating detonation engines (RDEs) are reported with stoichiometric hydrogen/air mixtures using Navier-Stokes equations with non-slip walls. The effects of isothermal wall boundary conditions at 300 K, 600 K and 900 K are investigated and further compared with the simulation under adiabatic walls. With isothermal walls of 300 K, axial detonation is formed, which further leads to the quenching of detonation. With 600 K isothermal walls, the detonation wave has a similar stably propagating process as adiabatic walls. However, due to the wall heat loss, a lower detonation velocity is obtained in the simulation with isothermal walls. With 900 K isothermal walls, it is found that detonation quenches at first due to the high temperature of the walls, then re-initiates as a result of the collisions of shock waves. In addition, the temperature and heat flux distributions in the outer wall are analyzed. Both the peak temperature and heat flux are found to appear at the detonation wavefront. The averaged heat flux in the detonation region is about three times of the averaged heat flux evaluated in the outer wall. Moreover, the averaged temperature and heat flux remain nearly unchanged in the post-detonation region. Our results are in agreement with experimental studies and provide some insights for RDE thermal management.

Laser-Induced Ignition and Combustion Behavior of Individual Graphite Microparticles in a Micro-Combustor

Microscale combustion has potential application in a micro power generator. This paper studied the ignition and combustion behavior of individual graphite microparticles in a micro-combustor to explore the utilization of carbon-based fuels at the microscale system. The individual graphite microparticles inside the micro-combustor were ignited by a highly focused laser in an air flow with natural convection at atmospheric temperature and pressure. The results show that the ignition of graphite microparticles was heterogeneous. The particle diameter had a small weak effect on ignition delay time and threshold ignition energy. The micro-combustor wall heat losses had significant effects on the ignition and combustion. During combustion, flame instability, photophoresis, repetitive extinction and reignition were identified. The flame structure was asymmetric, and the fluctuation of flame front and radiation intensity showed combustion instability. Photophoretic force pushed the graphite away from the focal point and resulted in extinction. Owing to large wall heat loss, the flame quickly extinguished. However, the graphite was inductively reignited by laser.

Dynamics and stability of premixed hydrogen-air flames in square microchannels with wall temperature gradients

The dynamics and stability of premixed hydrogen-air flames in square microchannels with heated walls were investigated through three-dimensional direct numerical simulations. The inlet velocity and equivalence ratio were 1.5 m/s and 0.5. The effect of the wall temperature gradient characteristics on the flame dynamics and stability was examined varying the width and location of the wall temperature gradient for a channel height of 1.5 mm. Five distinct flame modes were observed at different wall temperature profiles: flame with repetitive extinction-ignition (FREI), pulsating flame, laterally oscillating flame, spinning flame, and steady flame modes. Furthermore, transitions between these flame modes were observed for specific inflow and boundary conditions. The effect of the channel height on the flame stability was investigated by varying the channel height from 1.0 mm to 1.677 mm for a fixed wall temperature gradient. As the channel height was increased, four of the flame modes, namely, FREI, laterally oscillating flame, spinning flame, and steady flame modes appeared sequentially. To determine whether this sequential appearance was associated with the variation of the wall heat loss, the maximum wall temperature was changed by small amounts. For a lower wall temperature, the laterally oscillating flame mode transitioned to the FREI mode, and for a higher wall temperature, unstable flame modes such as the FREI and laterally oscillating flame modes disappeared, resulting in stable flame.

Influence of wall heat loss on the emission characteristics of premixed ammonia-air swirling flames interacting with the combustor wall

The influence of wall heat loss on the emission characteristics of ammonia-air swirling flames has been investigated employing Planar Laser-Induced Fluorescence imaging of OH radicals and Fourier Transform Infrared spectrometry of the exhaust gases in combustors with insulated and uninsulated walls over a range of equivalence ratios, ϕ, and pressures up to 0.5 MPa. Strong influence of wall heat loss on the flames led to quenching of the flame front near the combustor wall at 0.1 MPa, resulting in large unburned NH3 emissions, and inhibited the stabilization of flames in the outer recirculating zone (ORZ). A decrease in heat loss effects with an increase in pressure promoted extension of the fuel-rich stabilization limit owing to increased recirculation of H2 from NH3 decomposition in the ORZ. The influence of wall heat loss resulted in emission trends that contradict already reported trends in literature. NO emissions were found to be substantially low while unburned NH3 and N2O emissions were high at fuel-lean conditions during single-stage combustion, with values such as 55 ppmv of NO, 580 ppmv of N2O and 4457 ppmv of NH3 at ϕ = 0.8. In addition, the response of the flame to wall heat loss as pressure increased was more important than the effects of pressure on fuel-NO emission, thereby leading to an increase in NO emission with pressure. It was found that a reduction in wall heat loss or a sufficiently long fluid residence time in the primary combustion zone is necessary for efficient control of NH3 and N2O emissions in two-stage rich-lean ammonia combustors, the latter being more effective for N2O in addition to NO control. This study demonstrates that the influence of wall heat loss should not be ignored in emissions measurements in NH3-air combustion, and also advances the understanding of previous studies on ammonia micro gas turbines.

Large-eddy simulation of hydrothermal flames using extended flamelet/progress variable approach

The primary purpose of this paper is to develop high-fidelity combustion models for hydrothermal flames. Based on the flamelet/progress variable (FPV) model, comprehensive real-fluid models are incorporated to treat the thermodynamic and transport properties. To account for the heat loss effects, the FPV model is extended to be non-adiabatic with the heat release damping (HRD) approach. Combined with a presumed probability density function closure, the model is then evaluated in laboratory-scale hydrothermal flames in the context of large-eddy simulation. Results show that the prediction from the FPV model is better than those from the EDM and EDC-MTS models, since it correctly predicts the flame lift-off height as well as the maximum temperature position. The wall heat loss has significant influence on the temperature and species distributions. The use of the non-adiabatic FPV approach improve considerably the predictions of the flame temperature by comparing with the experimental data.

Investigation of flame characteristic in porous media burner with pores step distribution in radial direction

In recent years, attention has been focused on the use of porous materials to enhance the efficiency of combustion systems and reduce the pollutants emission. However, the porous media burners have the problem of inclined flame when used for surface heat treatments. In order to solve this problem, a new structure of porous media burner with pores step distribution in radial direction is proposed in this paper, four representative pores distributions in radial direction are chosen in order to reveal the influences of the new structure on flame characteristic of the porous media burner. Results show that the radial distribution of pores in combustion zone of burner affects the flame characteristic greatly. In the pore concentrated region, the increase of resistance loss reduces the gas flow rate, and causes the temperatures of gas and solid lower than that in the other regions. Combustion in porous media burner with radial pores concentrated gradually from the centre to the wall is the optimal structure in the four cases. Compared with the other cases, the optimal structure can reduce the temperature difference between the wall and environmental, and then reduce the wall heat loss, increase the thermal efficiency of the burner, improve the flame distribution characteristics, reduce the pore blocking and the possibility of structural failure, and increase the service life of the burner.

Effects of multilayer porous ceramics on thermochemical energy conversion and storage efficiency in solar dry reforming of methane reactor

In solar thermochemical systems, the utilization of porous ceramics plays an important role in the enhancement of heat transfer and optimization of reaction conditions, thereby effectively improving the energy conversion and storage efficiency. Compared with the common filling pattern of one-layer porous ceramic (1-LPC), novel changes in the thermal and chemical characteristics can be induced using multilayer porous ceramics (MPCs). To determine whether MPCs have advantages over 1-LPC in solar thermochemical applications, a numerical model was established in this study by combining computational fluid dynamics with dry reforming of methane reaction kinetics. The local thermal non-equilibrium model coupled with the P1 approximation was adopted to solve the solar radiation diffusion and convective heat transfer problems, while the non-Darcy flow effect was considered to predict the momentum dissipation resulting from the porous ceramics. Based on this, the effects of layer number, gap position, porosity, and cell size were investigated to find the optimal application strategies for MPCs. The simulation results indicate that a large temperature gradient in the first gap between two layers of MPCs can usually reduce the wall heat loss and improve the thermal efficiency, but has no universal effect on improving the solar-to-fuel efficiency. Under the current operational conditions, although improvement of the solar-to-fuel efficiency by approximately 0.03%–2.43% can be obtained using a 4-LPC in the cases of high porosities (ϕ⩾0.86) and large mean cell sizes (dp⩾7mm), 1-LPC remains the most reliable filling pattern with a wider range of applications and stable performance.

Flame Propagation and Oscillation in a Millimeter-scale Constant Volume Space

ABSTRACT Flame propagation pictures of a propane/air mixture are captured by a high-speed camera in a visualized constant volume combustion chamber with a diameter of 150 mm, a parallel plate height of H = 1.45 mm, and an initial pressure of P 0 = 1.25 ~ 3.00 bar, in the range of flammable equivalence ratios. The results show that when H of the combustion chamber decreases, the range of the flammable equivalence ratios greatly decreases. As the equivalence ratio increases, the flame propagation speed first increases and then decreases. Due to the combined effects of the wall surface and heat loss, the propagation speed is maximum at a rich mixture of approximately φ = 1.4. As the initial pressure increases, the flame propagation speed increases. The initial pressure has an enhancing effect on flame propagation in small space. At φ = 1.5 ~ 1.6, under a suitable initial pressure, the phenomenon of periodic stagnation of flame propagation is observed. The instability in this flame propagation is caused by the combination of the wall surface, Darrieus-Landau instability, diffusive-thermal instability and Taylor instability. The Taylor instability and initial pressure play important roles in the periodic oscillation of unstable flame propagation. However, the initial pressure can change only the amplitude of the oscillation, not the frequency.

Experimental investigation on natural convective heat loss of cylindrical cavity with/without a quartz window

This paper presents an experimental setup for the study of the natural convective heat loss characteristics of a cylindrical cavity with/without a quartz window. The thermal performance such as wall temperature and heat loss of two types of cavity receivers are studied by changing the tilt angles and heated states of the walls, then the effect of quartz window on the heat loss of the cavity is highlighted. The average temperature of cavity with a quartz window is respectively 68.61 °C, 79.36 °C, 48.22 °C higher than same cases without a quartz window (corresponding tilt angle is −90°, −30° and 45°). The temperature difference is within 10 °C for quartz window cavity and 40 °C for no quartz window cavity when q = 700 W/m2 and all surfaces are heated. More than 70% cases occur that the natural convective heat loss of no quartz window cavity is greater than that of the quartz window cavity. The experimental results show that the quartz window can significantly increase the operating temperature of the cavity and weaken the influence of tilt angle. The quartz window plays an important role in the balance of cavity temperature and reduction of natural convective heat loss. The empirical correlations of predicting the radiant and natural convective Nusselt numbers of quartz window cavity considering tilt angle and heat flux have also been obtained.

Energy investigation framework: Understanding buildings from an energy perspective view

A careful design and construction of energy efficient buildings is essential to reduce both the global energy consumption and carbon dioxide emissions. The desired outcome can be achieved by increasing the construction of low energy buildings, which are designed to sustain desired indoor temperature with a minimal energy input, and by improving the energy performance of existing buildings. Some buildings underperform by gaining or losing more heat than needed. This study introduces a framework for investigating building energy performance. Thermography investigation, building modelling, characterization of thermal bridges and future prediction for overheating are encapsulated in the proposed framework. A sport changing facility, which was designed as a low energy building, serves as a demonstrator for the application of the framework. The energy investigation framework revealed that the facility is underperforming. According to the building model, the main reason for the poor building performance is thermal bridging (presence of steel members), which increases gas consumption and wall heat-loss by 18% and 11%, respectively. Other contributors to heat lose/gain are cracks in the building envelop, weak mortar joints and uninsulated hot water pipes. Furthermore, the future temperature data, which is input to the building model, suggests that the entire facility is under the risk of overheating.