Wall Emissivity Research Articles

Numerical study on infrared radiation intensity and suppression methods of nozzle considering multi-component effects

Improving the accuracy of infrared intensity calculation in exhaust system and expanding infrared suppression methods are of great significance for enhancing aircraft stealth performanc. In this paper, the infrared radiation (IR) intensity (3–5 µm) of nozzle considering the influence of turbine and afterburner are numerically studied. Initially, the flow field characteristics are computed using commercial software, followed by ray tracing simulations using the reverse Monte Carlo method (RMCM) and transmission calculations using the multi-scale multi-group wideband k distribution model (MSMGWB). The wall emissivity is determined experimentally. This paper quantitatively analyzes the difference between infrared intensity obtained through traditional computational methods and that of the actual model, revealing the impact of low-emissivity coating and turbine cooling on the infrared intensity of the integrated model. It emphasizes the suppression effects and physical mechanisms of the heat shield flanging structure on the infrared characteristics of the integrated model. The results indicate significant differences between infrared intensity derived from traditional computational methods and that from actual models. The influence of the afterburner and turbine infrared characteristics must be considered in infrared numerical studies. When the computation model is a standalone nozzle, the infrared intensity can increase by up to 541.78 % and decrease by a maximum of 14.02 % compared to the integrated model compose of the turbine, afterburner, and nozzle. For the model composed of nozzle and afterburner, the infrared intensity can decrease by up to 10.5 % under non-swirl flow condition, without any increase observed. Compared with turbine wall, changing the emissivity of afterburner wall has more significant effect on infrared suppression of integrated calculation model. Reducing the emissivity of all afterburner walls from 0.7 to 0.1 significantly reduces the infrared characteristics in the range of 0° to 12.5°, while increasing it in the range of 12.5° to 45°. Applying a low-emissivity coating only to the high-temperature walls of the afterburner can inhibit the increase in emitted radiation on the wall, thereby eliminating the increase in infrared intensity mentioned above and reducing the infrared intensity by up to 39.14 %. Compared to low-emissivity coating technology for turbine, implementing cooling measures on turbine blades can effectively reduce the infrared intensity of the integrated model, achieving a maximum reduction of 19.7 % when the blade temperature is lowered by 240 K. Adding flanging structure to the heat shield in the convergent section alters the flow field structure near the nozzle walls, lowering wall temperature and thereby suppressing the infrared characteristics of the integrated model between 10° and 80°. This results in a maximum reduction in infrared intensity of 14.75 % with negligible impact on thrust coefficients. This is the angle range where afterburner low emissivity coatings and turbine cooling cannot achieve infrared suppression. This is of great significance for infrared stealth in large angle range. Combined with the above effective infrared suppression measures, the infrared intensity can be reduced by up to 49.42 %.

Study on radiation heat transfer characteristics of aeroengine combustion chamber

The thermal radiation of gaseous combustion products in an aero-engine combustion chamber has obvious non-ash-body characteristics. In this paper, based on the OpenFOAM platform, a numerical study on multi-field coupled turbulence-combustion-radiation radiative heat transfer was carried out for a certain type of aero-engine combustion chamber using the mean weighted sum of grey gas (WSGG) gas radiation model, which can accurately describe the non-ash characteristics of the gas. The results show that the radiation heat transfer characteristics are influenced by several factors such as equivalent ratio, combustion pressure and wall emissivity, among which the equivalent ratio is dominant. Increasing the equivalence ratio from 0.11 to 0.4 resulted in a 189% increase in average incident radiation at the combustion chamber wall and a 233% increase in average incident radiation at the exit. Increasing the equivalent ratio, combustion pressure or wall emissivity will increase the average radiation intensity in the combustion chamber and the inhomogeneity of radiation distribution at the exit of the combustion chamber. The radial radiation distribution factor (RRDF) also increases with the increase of equivalent ratio, combustion pressure and wall emissivity.

Influence of an inclined thick partition on free convection heat transfer performance under surface radiation in a hollow block

In the present work, a computational analysis of the combined energy transfer by free convection, radiation, and heat conduction in 2D hollow block with an inclined partition was carried out. The inclined partition was made of a heat-conducting material and had a finite thickness. The closed space inside the brick was filled with air (Pr = 0.71) and the airflow is laminar. The external surfaces of the vertical borders are considered to be isothermal, and the remaining horizontal surfaces are supposed to be adiabatic. The finite difference technique is employed to work out numerically the differential equations. The in-house computational code was verified in detail using various problems. The major characteristics that determine the considered phenomena are surface emissivity of internal walls, Ra number, and material of solid walls and partition. As a result of the research, the distributions of streamlines and isotherms combined with the average Nusselt numbers were obtained. Depending on the material of the partition, the flow structure in the cavity changes significantly, which reflects the distribution of streamlines, namely, for the materials with low thermal conductivity the convective cell in the upper part of the cavity is elongated, while in the lower part it is shifted to the lower and left borders. The isotherms reflect the more intensive heating in the left part of the area. With an increase in the thermal conductivity coefficient, the streamlines reflecting the nature of the flow are restructured. It was shown that the presence of an inclined partition significantly reduces the intensity of convective heat transfer.

Inner walls or vortices? Crescent-shaped asymmetries in ALMA observations of protoplanetary discs

ABSTRACT Crescent-shaped asymmetries are common in millimetre observations of protoplanetary discs and are usually attributed to vortices or dust overdensities. However, they often appear on a single side of the major axis and roughly symmetric about the minor axis, suggesting a geometric origin. In this work, we interpret such asymmetries as emission from the exposed inner cavity walls of inclined discs and use them to characterize their vertical extent. Here we focus on the discs around CIDA 9 and RY Tau, first modelling their observations in visibility space with a simple geometric prescription for the walls, and then exploring more detailed radiative transfer models. Accounting for the wall emission yields significantly better residuals than purely axisymmetric models, and we estimate the dust scale height of these systems to be 0.4 au at 37 au for CIDA 9 and 0.2 au at 12 au for RY Tau. Finally, we identify crescent-shaped asymmetries in twelve discs, nine of which have constraints on their orientation – in all cases, the asymmetry appears on the far-side of the disc, lending support to the hypothesis that they are due to their inner rims. Modelling this effect in larger samples of discs will help to build a statistical view of their vertical structure.

Thermal design of insulation on the outside of the cavity for a Trombe wall with phase change materials

A Trombe wall with a phase change material (TW-PCM) is a practical and low-carbon passive solar heating method. However, the heat efficiency is not high due to the large amount of heat loss at night. Insulation on the outside of the cavity is a suitable solution, but the design method has not been obtained, which hinders the realization of zero energy buildings. In the present study, we analyzed the thermal performance of a TW-PCM with insulation on the outside of the cavity at nighttime to develop the design approach. A mathematical model was established and validated based on experiments. This model was used to analyze the heat efficiency in different outdoor climates for a TW-PCM with insulation on the outside of the cavity. As well, the solar heating fraction (SHF) was analyzed. The results indicated that insulation on the outside of the cavity could significantly improve the thermal performance due to the reduced heat loss during the nighttime. The recommended additional thermal resistance due to the insulation on the outside of the cavity was negatively correlated with the average outdoor air temperature in January based on the analysis of various climates in China. The thermal resistance of the closed air cavity, formed by the TW-PCM and the insulation component, at the same wall emissivity was similar under the recommended additional thermal resistance in different outdoor climates. The SHF was improved significantly for the TW-PCM buildings with insulation on the outside of the cavity and it had a linear relationship with the solar heating load ratio (SLR).

A comprehensive heat transfer prediction model for tubular moving bed heat exchangers using CFD-DEM: Validation and sensitivity analysis

This study established a complete heat transfer prediction model that integrates four heat transfer sub-models, including contact heat conduction, gas film heat conduction, heat radiation, and heat convection, along with an artificial softening correction based on combined approach of computational fluid dynamics and discrete element method (CFD-DEM). The model’s precision was confirmed by experimental results with relative errors below 5 %. The discussion on the heat flux contributions of the above four heat transfer mechanisms shows that with an initial particle temperature of 200℃, tube wall temperature of 30℃, and particle descent velocity of 1.3 mm/s, gas film conduction heat flux is the primary contributor to the total tube wall heat flux at 60.5 %, followed by radiation heat flux (19.5 %), contact conduction heat flux (12.1 %), and convection heat flux (7.9 %). Besides, the presentation of physical fields related to particles and the fluid showcases the model’s capability in elucidating the heat and mass transfer characteristics of the particle–fluid-wall system within tubular MBHEs. Furthermore, a local nominal range sensitivity analysis, along with a study of factors’ influence rules, was performed to quantify the effects of the 8 key model parameters on heat transfer results. The analysis indicates that fluid thermal conductivity is the predominant factor with its nondimensionalized first-order sensitivity coefficient of 54.9 %, succeeded by particle-to-wall angle factor (15.4 %), wall emissivity (14.4 %), gas film thickness (9.6 %), wall thermal conductivity (2.3 %), wall roughness (-1.6 %), particle Young’s modulus (-1.4 %), and particle specific heat capacity (0.4 %). Finally, their impacts on the heat prediction results were presented in detail.

Kinetic modeling of the plasma–wall interaction in the DTT divertor region

A precise estimate of the local energy fluxes and erosion profiles at the divertor monoblocks of a fusion reactor requires a kinetic modeling of the plasma–wall interaction. Here, a two-dimensional Particle-in-Cell code is used to quantify the particle and energy fluxes and ion impact distribution functions across the divertor monoblocks of the ‘Divertor Tokamak Test’ reactor, focusing on poloidal gaps with toroidal bevelling. The considered critical locations are close to the strike points at both Inner and Outer Vertical Targets. A worst-case scenario for particle fluxes corresponding to attached plasma conditions and featuring a single-null magnetic configuration is assumed. The separate and cumulative effects of including electron wall emission and ions/electrons collisions with a background neutral gas (recycled at the walls) are also assessed. It is found that a non-negligible energy flux affects the shadowed regions of the monoblocks, especially when accounting for collisions, and that the ion impact distribution functions are strongly influenced by the considered kinetic effects, with important implications on the induced sputtering yield.

LBM simulation for combined thermal radiation and natural convection in 2D enclosure with multiple solid blocks

A numerical investigation of conjugate natural convection in a 2D square enclosure with multiple solid blocks, considering the impact of volumetric thermal radiation, was conducted using the lattice Boltzmann method (LBM). The effect of Planck number wall surface emissivity extinction coefficient solid-to-fluid volume fraction and the number of solid blocks on the fluid-structure and heat transfer within the enclosure were analyzed. The numerical model was validated using benchmark problems from the literature, showing good agreement. The results revealed that at and increasing wall surface emissivity significantly increased the Nusselt number, ranging from approximately 51-116% for different wall emissivity values, as compared to scenarios without surface radiation (). On the other hand, reducing χ, Pl, and β improved the heat transfer rate. The results also demonstrated that subdividing the initial block reduced the heat transfer rate depending on the number of blocks and the value of the Planck number. For instance, when 64 solid blocks were present in the cavity and the Nusselt number decreased by 59.36% compared to the case with a lower solid-to-fluid volume fraction (χ = 15%). The parametric analysis provided useful insights for engineering design in the modern construction industry, with possible applications in building block design and energy efficiency enhancements.

Influence of dimension variations of a fin and wall emissivity on the nanofluids flow inside a square cavity using the two-phase Lattice Boltzmann method

One of the numerical methods to study nanofluids (NFs) flow is the two-phase Lattice Boltzmann method (LBM). In this research, the NFs flow inside a square cavity (SQC) is examined using the LBM. The two cases of NFs flow inside a SQC with a fin are studied. The walls are at a low temperature on the left and right sides of the chamber, and the fin at the center of the chamber is always at a high temperature. The fin is placed horizontally and vertically and its effect on the flow is assessed by changing its fin length (L-F). The radiation (RD) equations are considered, while the horizontal walls of the SQC are insulated. It is found that the heat transfer (HTR) is enhanced with the L-F for both horizontal and vertical situations. For the vertical fin (VFN), the vortices are stronger than that for the horizontal fin (HFN), leading to more HTR. On the other hand, as the emissivity of the walls is intensified, the Nusselt number (Nu) is reduced, the RD Nusselt number is enhanced, and the overall Nu is increased.

Two-dimensional collisional particle model of the divertor sheath with electron emissive walls

A novel two-dimensional particle-in-cell (PIC) code, named Divertor Edge Simulator of Plasma-wall Interaction with Consistent COllisions (DESPICCO) and developed at CNR-ISTP, is capable of simulating the thin plasma layer of several millimeters, adjacent to the divertor tiles of a Tokamak fusion reactor. Here, kinetic effects and non-neutral plasma physics in the Debye sheath can be self-consistently captured by the PIC approach. The code is firstly benchmarked against literature one-dimensional codes and additional theoretical predictions for a magnetized sheath. Then, it is applied to a realistic divertor scenario featuring an attached plasma with monoblocks (MBs) radial misalignment and gaps, to compute the energy flux amplification factor at the exposed MB edge. A non-ambipolar local current density close to the leading edge and an average sheath heat transmission coefficient larger than the one predicted by classical sheath theory, are found. The effects of electron wall emission and plasma-gas collisions on the ion Mach number and on particle and energy fluxes to the walls are finally estimated to determine future guidelines for simulations. Ion collisions with recycled neutrals and both secondary and thermionic electron emission from the wall are found to have a relevant impact, with the overall effect of reducing by 25% the average ion impact energy, and by 15%–20% the total heavy particles energy flux to the walls, with relevant implications on the divertor wall erosion.

Inverse analysis of radiative properties of internal medium and surface for cylindrical system using CSM-CGM approach

An inverse analysis is conducted to simultaneously estimate the radiative properties in a 2D cylindrical system including the extinction coefficient, scattering albedo, and the wall emissivity from the knowledge of the radiative heat flux over different boundaries. In which the collocation spectral method (CSM) is developed to solve the forward problem, the conjugate gradient method (CGM) is employed to solve the inverse problem, and the complex-variable-differentiation method is used to treat the sensitivity problem, separately. Meanwhile, the effects of measurement errors, initial values, and imaginary values of complex variables on the precision of the inverse analysis are investigated. Results show that the proposed inverse analysis can successfully retrieve the unknown radiative physical properties of 2D cylindrical system. Under the same physical properties, good inversion results can be obtained by using different wall (end walls and side wall) radiative heat fluxes, but the inversion efficiency from sidewall radiative heat flux is higher. Besides, it is found that, the inversion for the scattered participating medium is more efficient than that for the non-scattered participating medium; the true values and iterative convergence values have little effect on the inversion result; the influence of the initial values and the imaginary values of the complex variable on the inversion cannot be ignored.

Heat transfer enhancement by inserting a radiation enhancement plate in turbine blade internal cooling channel

A novel method of inserting a radiation enhancement plate in a turbine blade internal cooling channel is established in this paper. It can extend cold surfaces for radiation and thus enhance total heat transfer. Numerical investigations are carried out to explore the flow and coupled convection and surface radiation in the rotating channel for different channel models. The results indicate that the model inserting the radiation enhancement plate roughed with W-shaped ribs (model D) exhibits the highest cooling performance. Compared with the existing channel model (model A), the total Nusselt number ratio and the performance evaluation criterion increase by 90.53% and 21.67%, respectively, and the maximum and average temperature decrease by 339.80 K and 73.07 K, respectively. In addition, the heat transfer difference between the leading and trailing walls caused by the Coriolis force is remarkably reduced. Influences of pertinent parameters including the Reynolds number, the rotation number, the wall heat flux, and the wall emissivity are also investigated. It is demonstrated that the method has an excellent self-enhancement effect.

Variable discrete ordinates method for radiation transfer in concentric spherical variable index media

Radiation heat transfer in participating concentric spherical media with variable refractive index along the radial direction is solved by the variable discrete ordinates method. The radiative properties, including refractive index, are assumed to be constant in each volume element, such that a radiative ray travels along straight line in each volume element, and redirects by passing the interface of volume elements according to Snell's law of refraction, or specularly reflects back into the volume element. The radiative transfer equation in spherical coordinates system is solved for different number and direction of ordinates in each volume element. The performance of the present method is examined for two cases with medium's specified temperature and radiative equilibrium with continuous linear, continuous non-linear, and discontinuous profiles of refractive index. The accuracy of the present method is checked by comparing with benchmark solutions. For the cases where the exact solution is not available, the energy conservation criterion is used to validate the solution method. The effects of radiative properties, such as wall emissivities, optical thickness, single scattering albedo, and anisotropy factor are examined by some example problems.

Improved Monte Carlo method for radiative heat transfer in semitransparent media with BRDF surface

This paper proposes an improved Monte Carlo method (IMCM) for the radiative heat transfer solution in semitransparent media with BRDF surface for the first time. This method not only keeps the strong points of high precision and excellent applicability to the calculation of radiative heat transfer in complex shape scattering media with BRDF surface, but also overcomes the shortcomings of time-consuming for huge amounts of energy beams tracing in the Monte Carlo method. In the IMCM we developed, the energy beams of reflection or scattering can be worked out by proportional iteration accumulation. The scattering and reflection process only need to be sampled twice rather than lots of scattering and reflection energy beams tracing. This improvement not only retains the high accuracy of the MCM, but also greatly improves the calculation efficiency. A number of radiative heat transfer problems in semitransparent media enclosure with BRDF surface are studied. The effects of absorption coefficient, wall emissivity and scattering characteristics on radiative heat transfer are analyzed. Results indicate that the IMCM is a very efficient method with high precision for solving radiative heat transfer.

Wall emissivity solved by bidirectional reflection distribution function combined with Bi-LSTM network

Wall emissivity solved by bidirectional reflection distribution function combined with Bi-LSTM network

Integrated use of CFD and field data for accurate thermal analyses of oil/gas boilers

In the present study we develop a complete Computational Fluid Dynamics (CFD) modeling procedure suitable for accurate simulations of industrial boilers fed alternatively with gaseous and liquid fuels. The model is developed and validated by means of data from on-site testing.Two different boilers are considered: a 6-burner steam generator of a refinery for the model definition, and a 32-burner thermal power plant boiler for validation, fed by flare-gas/natural-gas or Heavy-Fuel-Oil (HFO). Selected reliable experimental data coming from performance testing are used for both the set-up of the CFD simulations as well as to compute the Flue Exit Gas Temperatures (FEGTs) employed as validation criteria. The flare-gas testing on the 6-burner boiler allows us to accurately determine that a membrane wall emissivity around 0.60 is appropriate as a boundary condition for radiation. By a sensitivity analysis, the relevance of wall emissivity with respect to fouling thermal resistance on the overall heat transfer inside the furnace is established. For boilers alternatively operating with gaseous fuel and HFO a value of 2.8 kW/(m2·K) has been found appropriate. HFO testing on the 6-burner boiler provides data to develop the additional soot modeling, crucial to properly catch oil flame emissivity, based on the model of Khan and Greeves. This allows the matching of a mean soot mass fraction of 1%–3% in the flame zone, a range found by in-flame measurements in the literature and confirmed in the present study by overall heat exchange data in the whole furnace coming from HFO boiler testing. Finally, the 32-burner boiler is considered for validation: using the same procedure and parameters, excellent agreement is found between experimental and CFD results, both on a 100% load natural gas and on a 107% load HFO performance testing.The study demonstrates how field data can be used to validate CFD simulations and confirms that the use of physically meaningful parameters and models exempts from repeated tuning: no change is needed for emissivity and fouling thermal resistance between gaseous and liquid fuel operation, and even a simple soot modeling can be used, just ensuring physically consistent particulate concentration in the oil flame.

Demonstration of Natural Gas and Hydrogen Cocombustion in an Industrial Gas Turbine

Abstract Hydrogen cofiring in a gas turbine is believed to cover an energy transition pathway with green hydrogen as a driver to lower the carbon footprint of existing thermal power generation or cogeneration plants through the gradual increase of hydrogen injection in the existing natural gas grid. Today there is limited operational experience on cocombustion of hydrogen and natural gas in an existing gas turbine in an industrial environment. The ENGIE owned Siemens SGT-600 (Alstom legacy GT10B) 24 MW industrial gas turbine in the port of Antwerp (Belgium) was selected as a demonstrator for cofiring natural gas with hydrogen as it enables ENGIE to perform tests at higher H2 contents (up to 25 vol. %) on a representative turbine with limited hydrogen volume flow (one truck load at a time). Several challenges like increasing risk of flame flashback due to the enhanced turbulent flame speed, avoiding higher NOx emissions due to an increase of local flame temperature, supply and homogeneous mixing of hydrogen with natural gas as well as safety aspects have to be addressed when dealing with hydrogen fuel blends. In order to limit the risks of the industrial gas turbine testing a dual step approach was taken. ENGIE teamed up with the German Aerospace Center (DLR), Institute of Combustion Technology in Stuttgart to perform in a first step scaled-burner tests at gas turbine relevant operating conditions in their high-pressure combustor rig. In these tests the onset of flashback as well as the combustor characteristics with respect to burner wall temperatures, emissions and combustion dynamics were investigated for base load and part load conditions, both for pure natural gas and various natural gas and hydrogen blends. For H2 < 30 vol. % only minor effects on flame position and flame shape (analyzed based on OH* chemiluminescence images) and NOx emission were found. For higher hydrogen contents the flame position moved upstream and a more compact shape was observed. For the investigated H2 contents no flashback event was observed. However, thermo acoustics are strongly affected by hydrogen addition. In general, the scaled-burner tests were encouraging and enabled the second step, the exploration of hydrogen limits of the second generation dry low emissions burner installed in the engine in Antwerp. To inject the hydrogen into the industrial gas turbine, a hydrogen supply line was developed and installed next to the gas turbine. All tests were performed on the existing gas turbine hardware without any modification. A test campaign of several operational tests at base and part load with hydrogen variation up to 25 vol. % has been successfully performed where the gas composition, emissions, combustion dynamics and operational parameters are actively monitored in order to assess the impact of hydrogen on performance. Moreover, the impact of the hydrogen addition on the flame stability has been further assessed through the combustion tuning. The whole test campaign has been executed while the gas turbine stayed online, with no impact to the industrial steam customer. It has been proved that cofiring of up to 10% could be achieved with no adverse effects on the performance of the machine. Stable operation has been observed up to 25 vol. % hydrogen cocombustion, but with trespassing the local emission limits.

Three-dimensional simulation of full conduction-convection-radiation coupling with high Rayleigh numbers

The present research is focused on numerical analysis of heat transfer and fluid flow patterns when taking into account conduction, natural convection and surface thermal radiation under three-dimensional problem formulation. A hybrid mathematical model coupling the mesoscopic lattice Boltzmann equations with the macroscopic energy equation is built. The governing equations are solved in MATLAB. An in-house code developed in this work can be both run on central processing units and graphics processing units with almost no changes. Multiparameter analysis was performed when varying the Rayleigh number (Ra), surface emissivity, Biot number, conduction-radiation number, thermophysical properties and thickness of the solid walls. During numerical simulation, it was found that the vertical walls emissivity can be used as a tool to control thermal and flow behavior. In fact, three modes of heat transfer such as thermal stratification, thermal fluctuations and thermal plume were revealed when Ra = 108. With further increase of the Rayleigh number thermal and flow fluctuations were observed at the bottom half of the cavity. A significant discrepancy in results was observed between the 2D and 3D models when taking into account surface radiation. Finally, correlations for mean convective, radiative and effective Nusselt numbers were proposed.

Numerical study on the impact of wall structure on the thermal performance of double-channel porous solar wall

With the improvement of people’s living standards, they have higher requirements for indoor thermal comfort in the cold season. Solar wall utilizing solar energy for heating can reduce carbon emissions and achieve carbon neutrality. In the aspect of solar wall research, the influence of wall structure on the thermal performance of double-channel porous solar wall is limitedly investigated. In fact, the optimization design of wall structure is important for the thermal performance of solar wall and its applications. Therefore, a simplified three dimensional room model is built to study the influence of the wall structure on the thermal performance of porous solar wall by numerical simulation. With this model, different channel spacing and thickness of porous walls were used to determine the optimal design for a double-channel porous solar wall in terms of enhancing the heat storage. Moreover, the influence of the surface emissivity on the characteristics of heating and temperature field of double-channel porous solar wall are studied based on the optimal structure. The CFD simulation results indicate that the optimal structure parameters should include spacing of 0.08 m for channel 1, the porous wall thickness should be 0.08 m, and the air channel 2 spacing should be 0.06 m. The temperature of air channel 1 and air channel 2, the indoor temperature, and the heat storage of porous wall decrease with the increase of the surface emissivity of the porous wall. In order to improve the heat storage performance of double-channel porous solar wall, the outer surface of the porous wall should use a lower emissivity material. The outer surface emissivity of porous wall has a significant impact on the heat storage of the porous wall and little effect on the thermal storage wall. The temperature of porous wall is always higher than that of outdoor environment temperature.

Free convection heat transfer with surface radiation from infrared suppression system and estimation of cooling time

• The thermo-fluid characteristics of the full-scale infrared suppression (IRS) system used in the marine gas turbine as an exhaust system are numerically investigated here. • The combined impact of conduction, convection, and radiation on the cooling of the IRS system is intended to be explored. • Various relevant parameters are taken for the analysis, viz., Rayleigh number (Ra), number of funnels, funnel wall temperature, diameter ratio (DR), funnel overlapping (OL), and emissivity of the surface (ε). • A generalized correlation for Nusselt number is developed, considering surface emissivity and the number of funnels as an independent parameter that can be used for academic and industrial applications. • The novelty of the work lies in finding the heat transfer rate and cooling time of the IRS device with varying numbers of funnels (3 to 6 funnels) subjected to the combined effect of free convection and surface radiation. A numerical study is performed on Infrared Suppression (IRS) system having finite wall thickness to observe the combined effect of free convection and surface radiation. The main focus of this work is to obtain thermo-fluid characteristics of natural convection phenomena from the IRS system due to conjugate heat transfer and surface radiation. In addition, the time required to cool down the hot IRS system to atmospheric temperature is also estimated. For the numerical analysis, different governing equations (Navier-Stokes equation, energy equation, and radiation equation) are solved using the finite volume method (FVM) based solver of ANSYS Fluent v 15. Several pertinent parameters, viz., the number of funnels, Rayleigh number, inner surface temperature, geometric ratio, funnel overlapping, and surface emissivity, are varied to elucidate the heat transfer behavior. It is evident from the research that surface radiation has a significant influent on total heat transfer and should not be ignored. In addition, the total heat transfer rate rises with the number of funnels. Both non-dimensional induced mass flow rate and total heat transfer rate have maximum value for zero overlapping cases. At constant temperature contribution of radiative heat transfer varies from 10 to 38% with the rise in emissivity. The inclusion of radiation with convection lowers the cooling time significantly compared to convection alone.