Adiabatic Heat Transfer Research Articles

Determination of Local Heat Transfer Coefficients and Friction Factors at Variable Temperature and Velocity Boundary Conditions for Complex Flows

Transient conjugate heat transfer measurements under varying temperature and velocity inlet boundary conditions at incompressible flow conditions were performed for flat plate and ribbed channel geometries. Therefrom, local adiabatic wall temperatures and heat transfer coefficients were determined. The data were analyzed using typical heat transfer correlations, e.g., Nu=CRemPrn, determining the local distributions of C and m. It is shown that they are closely linked. A relationship lnC=A−mB is observed, with A and B as modeling parameters. They could be related to parameters in log-law or power-law representations for turbulent boundary layer flows. The parameter m is shown to have a close link to local pressure gradients and, therewith, near wall streamlines as well as friction factor distributions. A normalization of the C parameter allows one to derive a Reynolds analogy factor and, therefrom, local wall shear stresses.

Study on the impact of heat loss on premixed hydrogen/air deflagration in closed pipelines

This study developed a one-step chemical reaction model for hydrogen combustion and investigated the effect of heat loss on premixed hydrogen/air deflagration in closed pipelines using numerical simulations with the CFD code GASFLOW-MPI. The simulation results were compared and verified with experimental data. The numerical simulations accurately predicted the flame front structure, pressure, and flame tip position during the deflagration of premixed hydrogen/air. Furthermore, a comparison of adiabatic simulation and heat transfer simulation results revealed that large-scale vortices within the turbulent flow were closely linked to heat loss during the tulip flame stage. Simultaneously, heat loss reduced pressure and flame tip position by diminishing the flame wrinkling factor caused by thermodiffusive instability. Finally, convective heat transfer identified as the most critical factor contributing to heat loss during the deflagration of premixed hydrogen/air in closed pipelines. Specifically, during the tulip flame stage, heat loss was mainly due to convective heat transfer (73.8%∼76.3%), with radiation playing a minor role (23.7%∼26.2%). Therefore, accurate prediction of characteristic parameters during the deflagration of premixed hydrogen/air requires numerical simulations that account for both convective and radiative heat transfer.

Heat Transfer Coefficient and Adiabatic Effectiveness Measurements on a Nozzle Guide Vane With a Single Row of Cylindrical Holes

Abstract Film-cooling injection significantly affects the thermal behavior of turbine vane surfaces. In addition to the beneficial effect of the film shielding the vane from the hot gas flow, alteration of the thermal boundary layer should also be taken into account. The aim of the present work is to detail the film-cooling performance in terms of adiabatic effectiveness and external heat transfer coefficient on a 2D nozzle guide vane. A single row of cylindrical holes was tested on both pressure and suction sides of a literature vane, the VKI LS89, in a linear cascade. The employed measurement technique is a transient thermal method based on infrared thermography, which was thoroughly described and validated in a previous work. The influence of inlet freestream turbulence and blowing ratio was evaluated, and two different injection angles were considered for both pressure and suction sides. Spatially resolved distributions of adiabatic effectiveness and heat transfer coefficient (HTC) on the vane surface allow us to precisely quantify the above-mentioned aspects and highlight qualitative differences between pressure side and suction side behavior. Details regarding the generated non-uniformities in the measured parameters could be also provided, to emphasize how average quantities are not always sufficient to characterize such complex phenomena. The impact of different reference conditions to scale HTC results was also investigated. Such effect was found not negligible on the overall performance of the film-cooling system, especially on the suction side where transition plays a critical role. Ultimately, the collected results constitute a wide and detailed experimental database for numerical modeling validation in a well-studied environment as the LS89 configuration.

Erratum: “Comprehensive characterization of the adiabatic temperature change and (convection) heat transfer coefficient of a NiTi tube elastocaloric refrigerant” [AIP Adv. 13, 065202 (2023)

Erratum: “Comprehensive characterization of the adiabatic temperature change and (convection) heat transfer coefficient of a NiTi tube elastocaloric refrigerant” [AIP Adv. 13, 065202 (2023)

Effect of Microchannel on the Composite Cooling Performance of a Simulated Turbine Blade Leading Edge

Abstract As the turbine blade leading edge becomes one of the hottest regions in the engine, a microchannel structure was applied to further improve the cooling performance. Adiabatic and conjugate heat transfer analyses were conducted to compare the heat transfer and cooling performance of the leading edge with or without microchannels, while the influence of mass flowrate of coolant was also included. Based on the investigations on the effect of geometrical designs, the flow and composite cooling performance of the microchannels were further optimized. The results indicate that the internal heat transfer intensity was greatly enhanced with the arrangement of microchannels, while the adiabatic film coolant coverage was slightly deteriorated. On this basis, the composite cooling performance of the leading edge was effectively improved by at least 7.54%. However, the flow resistance of film cooling holes was obviously increased due to the obstruction of microchannels, and a larger thickness of microchannels would alleviate this feature. In addition, the composite cooling performance of the leading edge was further optimized by increasing the radius of microchannels, and the same effect was achieved by reducing the spanwise distance between the film cooling holes and the microjet holes. Compared with the leading edge model without microchannels, the former design increased the area-averaged overall cooling effectiveness of the leading edge by at least 11.17% under all the mass flow rates of coolant studied in this paper, while the latter design improved the composite cooling performance by at least 12.76%.

Comprehensive characterization of the adiabatic temperature change and (convection) heat transfer coefficient of a NiTi tube elastocaloric refrigerant

Elastocaloric cooling without global warming substance emission is a promising alternative to the vapor-compression technology. Comprehensive and precise characterization of the adiabatic temperature change of the elastocaloric refrigerant and the heat transfer coefficient between the elastocaloric refrigerant and the surrounding heat transfer fluid/solid is significant for the structural design of elastocaloric coolers. In this article, an analytical solution of the volume-averaged temperature variation in a tubular shape memory alloy elastocaloric refrigerant under cyclic compression was derived using lumped analysis, and a method of comprehensive characterization of the adiabatic temperature change (∆Tad) and (convection) heat transfer coefficient h for the elastocaloric refrigerant based on the analytical solution was proposed. A dimensionless number Ch was defined as the ratio of the latent heat release/absorption rate to the (convection) heat transfer rate, with which the characterization procedure including a series of nonlinear least-square regression tests and data selection criteria were established. The method proposed is applicable to any cross section geometry under both tension and compression, and it was validated using experimental data on a NiTi tube under sinusoidal force-controlled and reverse Brayton cyclic compressions and using existing experimental data of NiTi-based films, strips, and pillars under tension and compression in the literature. The ∆Tad characterized using the proposed method agreed with that using the reverse Brayton cyclic loading method within 5% (absolute value of 1 K). The effect of data selection sequence on the characterization of ∆Tad and h was investigated, and the results showed that proper starting points were significant for the convergence of ∆Tad and h.

Fluid flow and heat transfer characteristics of three-dimensional slot film cooling in an annular combustor

Film cooling methodologies in air-breathing gas turbines have been evolving for many decades. In the present work, a combined experimental and numerical parametric study is conducted to investigate the three-dimensional slot film cooling of an annular combustor. The experimental investigation is conducted to understand the fluid flow and heat transfer phenomenon of the film cooling and to validate the numerical study under laboratory conditions (low temperature and pressure). Transient infrared thermography is used to estimate both adiabatic film-cooling effectiveness (ηad) and heat transfer coefficient (h) simultaneously using a semi-infinite approximation method. The blowing ratios considered in the study are in the range of 0.5 to 5. The experimental results showed that film-cooling effectiveness (ηad) enhanced with an increase in the blowing ratio from 0.5 to 2 and deteriorated beyond BR = 2 due to the adverse effects of turbulence. A parametric study is conducted numerically to understand the effect of flow and geometrical parameters under actual engine conditions (high temperature and pressure). The parameters considered are slot Reynolds number (Res), slot jet diameter (d), slot jet pitch (p), lip taper angle (α), lip length (L), and slot jet injection angle (β). For the considered parameters, numerical results showed that the slot jet diameter (d) = 2 mm, dimensionless slot jet pitch (p/d) = 2, slot jet injection angle (β)=20o and dimensionless lip length (L/d) = 5.9 outperformed the other configurations due to the low turbulence and entrainment. Finally, an Artificial Neural Network-based mathematical model is developed that correlates the ηlat as a function of flow and geometrical parameters.

Analysis of step-heating thermography methods for defect depth prediction

In this study, five theoretical equations of specific characteristic peak times are proposed to predict the defect depth for step-heating thermography. These equations were derived for test samples with surfaces under adiabatic and convective heat transfer conditions, where, as demonstrated from experiments, the effect of convection on depth prediction can be significant for materials with low thermal conductivities. Two glass fiber reinforced plastic (GFRP) samples with flat-bottom holes at various depths were inspected by step-heating thermography, and the defect depths were predicted by these five methods. Based on the experimental results, the prediction accuracy and the performance of these depth prediction methods are discussed.

Analytic Solution to Steady Heat Conduction with Transverse Radiation Heat Transfer

A unique analytic solution to the problem of steady heat conduction with radiative heat transfer normal to the conduction heat flow is presented. The solution is unique, as it does not assume zero temperature of the surroundings and addresses all possible tip boundary conditions for in-space applications. The temperature profiles in the direction of heat conduction are produced for a constant-temperature boundary condition at the base and three different boundary conditions at the tip: adiabatic, constant temperature, and radiative heat transfer. Approximate solutions to the implicit exact solution are also developed. An extended solution that includes convection for the adiabatic-tip boundary condition is also presented and compares very well to experimental data.

Comparison of adiabatic and conjugate heat transfer models on near-wall region flows and thermal characteristics of angled effusion cooling holes

This paper aims to evaluate the accuracy and efficiency of different numerical simulation approaches on cooling performance for the angled effusion cooling holes applied in a micro gas turbine (MGT) combustor liner. The geometric structure processing methods of a 3-D multi-perforated plate are performed for conjugate heat transfer (CHT) and adiabatic models. Numerical approaches are verified by experimental data in terms of centerline film cooling effectiveness. The wall cooling film effectiveness, film thickness and discharge coefficient of the multi-perforated plate are studied and discussed under the condition of whether there is solid heat conduction. Results show that the adiabatic model cannot predict the veritable wall cooling effectiveness distribution on the inner wall surface and overestimates the discharge coefficient by nearly 16.5%. However, the accuracy of the adiabatic model in predicting the near wall flow field structure is not lower than that of the CHT model. Both of the two model can predict a film thickness of 4.1 mm at the same sampling position. Further studies indicate that the increases of flow direction β have a significant impact on the improvement of wall cooling effectiveness distribution in the spanwise direction. The Ux magnitude predicated by adiabatic model is increasingly close to that of CHT model. The predication results also illustrate a relatively fixed difference of 0.1 between these two models.

Coupled CFD-FEM Simulation of Steel Box Bridge Exposed to Fire

Increasing fire-induced bridge failures are demanding more precise behavior prediction for the bridges subjected to fires. However, current numerical methods are limited to temperature curves prescribed for building structures, which can misestimate the fire impact significantly. This paper developed a framework coupling the computational dynamics (CFD) method and finite element method (FEM) to predict the performance of fire-exposed bridges. The fire combustion was simulated in CFD software, Fire Dynamic Simulator, to calculate the thermal boundary required by the thermomechanical simulation. Then, the adiabatic surface temperatures and heat transfer coefficient were applied to the FEM model of the entire bridge girder. A sequential coupled thermomechanical FEM simulation was then carried out to evaluate the performance of the fire-exposed bridge, thermally and structurally. The methodology was then validated through a real fire experiment on a steel beam. The fire performance of a simply supported steel box bridge was simulated using the proposed coupled CFD-FEM methodology. Numerical results show that the presented method was able to replicate the inhomogeneous thermomechanical response of box bridges exposed to real fires. The girder failed due to the buckling of a central diaphragm after the ignition of the investigated tanker fire in no more than 10 min. The framework presented in this study is programmatic and friendly to researchers and can be applied for the estimation of bridges in different fire conditions.

Numerical investigation of heat transfer effects on combustion in a generic gas turbine burner

PurposeThis study aims to methodologically investigate heat transfer effects on reacting flow inside a liquid-fueled, swirl-stabilized burner. Furthermore, particular attention is paid to turbulence modeling and the results of Reynolds-averaged Navier–Stokes and large eddy simulation approaches are compared in terms of velocity field and flame temperature.Design/methodology/approachSimulations consist liquid fuel distribution using Eulerian–Lagrangian approach. Flamelet-Generated Manifold combustion model, which is a mixture fraction-progress variable formulation, is used to obtain reacting flow field. Discrete ordinates method is also added for modeling radiation heat transfer effect inside the burner. As a parametric study, different thermal boundary conditions namely: adiabatic wall, constant temperature and heat transfer coefficient are applied. Because of the fact that the burner is designed for operating with different materials, the effects of burner material on heat transfer and combustion processes are investigated. Additionally, material temperatures have been calculated using 1 D method. Finally, soot particles, which are source of luminous radiation in gas turbine combustors, are calculated using Moss-Brookes model.FindingsThe results show that the flow behavior is obviously different in recirculation region for both turbulence modeling approach, and this difference causes change on flame temperature distribution, particularly in the outer recirculation zone and region close to swirler. In thermal assessment of the burner, it is predicted that material of the burner walls and the applied thermal boundary conditions have significant influence on flame temperature, wall temperature and flow field. The radiation heat transfer also makes a strong impact on combustion inside the burner; however, luminous radiation arising from soot particles is negligible for the current case.Originality/valueThese types of burners are widely used in research of gas turbine combustion, and it can be seen that the heat transfer effects are generally neglected or oversimplified in the literature. This parametric study provides a basic understanding and methodology of the heat transfer effects on combustion to the researchers.

The effects of ejector adiabatic absorber on heat and mass transfer of binary nanofluid with heat transfer additives

This paper presents experimental results on the study of the effects of ejector adiabatic absorber on heat and mass transfer of binary nanofluid with heat transfer additives (2-ethyl-1-hexanol and gum Arabic). In this case, H2O/lithium bromide-alumina nanofluid was suggested due to a growing interest in absorption heat transfer working fluid for solar energy application. An experimental setup — ejector test rig — was designed to study the absorption, heat, and mass transfer rate as a result of refrigerant vapour mass flow entrained by the ejector adiabatic absorber. The study was carried out at different solution mass flowrate (0.051 to 0.17 kg/s) with three prepared sample solutions, which include pure LiBr solution, LiBr-Alumina nanofluid without heat transfer additives, and LiBr-Alumina nanofluid with heat transfer additives. The absorption rate, mass transfer coefficient, heat transfer rate, and heat transfer coefficient for the three samples were reported. On the other hand, the percentage enhancements for all the parameters — at a suitable flow rate of 0.085 kg/s — due to the addition of alumina without and with heat transfer additives were recorded. The absorption rate enhancements were 25% and 96%, the enhancement rates of mass transfer coefficient recorded were 20% and 82%, the heat transfer rate enhancements were 85% and 183%, and the heat transfer coefficient enhancements obtained were 72% and 156% with addition of alumina nanoparticles only and alumina nanoparticles with heat transfer additives respectively. Material mass balance analysis suggests that mass inflow in the ejector equals to the mass outflow from the ejector, indicating a complete absorption of the entrained refrigerant vapour beyond which falling film absorption can occur due to concentration. This article also presents experimental evidence of the capability of ejector as strong adiabatic absorber, heat, and mass transfer component, which were earlier reported using numerical models.

Adiabatic effectiveness and heat transfer measurements of simple and trenched cylindrical holes with backward injection

Recent studies have demonstrated that the simple cylindrical hole with backward injection, where the jets are injected reverse to the mainstream flow direction, outperforms its forward-injected counterpart at relative high blowing ratios. Meanwhile, trench is known to be of high cooling effectiveness with forward injection at high blowing ratios. Few studies addressed the combination of the two setups. The present study investigated the simple and trenched cylindrical hole configurations with either forward or backward injection by experiments. The injection angle was 30° and 150° for the forward and backward setup separately. The trench had a depth of 0.75D and the trench width was 1D and 2D. Adiabatic effectiveness was measured with PSP, and heat transfer augmentation was obtained with transient method using the IR camera. Detailed distributions were obtained. Blowing ratio was varied from 0.25 to 4.0. The density ratio was 1.0 or 1.5 in PSP measurements, and was 1.0 only for the heat transfer measurements. For the simple cylindrical hole with backward injection, the result confirmed the advantage of it at high blowing ratios, and also showed that its adiabatic effectiveness is highly uniform in the spanwise direction. Meanwhile, the backward-injected configuration had a much more stable cooling performance as the VR varied. It was further pointed out that the flow field of the simple backward-injected case is governed by the recirculated region downstream the hole exit. The narrow trench enhanced the effectiveness hugely along with forward injection, while the enhancement to the simple backward injection was quite limited. Moreover, the narrow trench performed far better than the wide trench. It was revealed that the key of the mechanism depends on the interaction of the jet with the downstream trench wall.

Purge flow effects on rotor hub endwall heat transfer with extended endwall contouring into the disk cavity

Efficiency improvements for gas turbines are strongly coupled with increasing turbine inlet temperatures. This imposes new challenges for designers for efficient and adequate cooling of turbine components. Modern gas turbines inject bleed air from the compressor into the stator/rotor rim seal cavity to prevent hot gas ingestion from the main flow, while cooling the rotor disk. The purge flow interacts with the main flow field and static pressure field imposed by the turbine blades. This complex interaction causes non-uniform and jet-like penetration of the purge flow into the main flow field, hence affecting the endwall heat transfer on the rotor. To improve the understanding of purge flow effects on rotor hub endwall heat transfer, an unshrouded, high-pressure representative turbine design with 3D blading and extended endwall contouring of the rotor into the cavity seal was tested. The measurements were conducted in the 1.5-stage axial turbine facility LISA at ETH Zurich, where a state-of-the-art measurement setup with a high-speed infrared camera and thermally managed rotor insert was used to perform high-resolution heat transfer measurements on the rotor. Three different purge flow rates were investigated with regard to hub endwall heat transfer. Additionally, steady-state computational fluid dynamics simulations were performed to complement the experiments. It was found that the local heat transfer rate changes up to ±20% depending on the purge flow rate. The main part of the purged air is ejected at the endwall trough location and swept towards the rotor suction side, which is caused by the interaction of main flow and the cavity extended endwall design. The presence of low momentum purge flow locally reduces the heat transfer rate. Changes in adiabatic wall temperature and heat transfer (depending on purge rate) are observed from the platform start up to the cross passage migration of the secondary flow structures.

Numerical assessment of round-to-slot film cooling performances on a turbine blade under engine representative conditions

Numerical simulations are carried out to address further insights into the effects of round-to-slot film cooling holes on a multi-row film-cooled turbine blade under representative engine-simulated conditions by adopting both adiabatic and conjugate heat transfer CFD models. In the current study, only the alteration of film-hole shape at suction and pressure sides is concerned while the showerhead film cooling holes at the leading edge remain cylindrical or baseline shape. Four film-hole shapes are taken into consideration, including converging round-to-slot hole (RTSH-1), equivalent-area round-to-slot hole (RTSH-2), diffusing round-to-slot hole (RTSH-3) and fan-shaped hole (FSH). Beside, a series of computations are also performed to the individual single-row film cooling by setting the coolant-feeding condition as its corresponding one in the multi-row film cooling situation. Viewed from the comprehensive influence of film-hole shape on film cooling performance, such as coolant feeding usage amount, enthalpy loss coefficient and cooling effectiveness, it is suggested that RTSH-2 should be the most favourable shape. RTSH-1 leads to the smallest coolant usage and enthalpy loss coefficient but its cooling effectiveness is low. Although RTSH-3 could present the highest cooling effectiveness, it produces the largest coolant usage and cascade enthalpy loss coefficient.

Experimental Evidence of Temperature Ratio Effect on Turbine Blade Tip Heat Transfer

Determination of a scalable Nusselt number (based on “adiabatic heat transfer coefficient”) has been the primary objective of the most existing heat transfer experimental studies. Based on the assumption that the wall thermal boundary conditions do not affect the flow field, the thermal measurements were mostly carried out at near adiabatic condition without matching the engine realistic wall-to-gas temperature ratio (TR). Recent numerical studies raised a question on the validity of this conventional practice in some applications, especially for turbine blade. Due to the relatively low thermal inertia of the over-tip-leakage (OTL) flow within the thin clearance, the fluids' transport properties vary greatly with different wall thermal boundary conditions and the two-way coupling between OTL aerodynamics and heat transfer cannot be neglected. The issue could become more severe when the gas turbine manufacturers are making effort to achieve much tighter clearance. However, there has been no experimental evidence to back up these numerical findings. In this study, transient thermal measurements were conducted in a high-temperature linear cascade rig for a range of tip clearance ratio (G/S) (0.3%, 0.4%, 0.6%, and 1%). Surface temperature history was captured by infrared thermography at a range of wall-to-gas TRs. Heat transfer coefficient (HTC) distributions were obtained based on a conventional data processing technique. The profound influence of tip surface thermal boundary condition on heat transfer and OTL flow was revealed by the first-of-its-kind experimental data obtained in the present experimental study.

Computational Study of Combustor Dilution Flow Interaction with Turbine Vanes

Higher efficiency and greater performance in gas-turbine engines can be achieved by increasing the combustion temperature but is limited by durability concerns for downstream hardware. Large-scale dilution cooling flows can be injected in the combustor to promote mixing and reduce incoming temperatures, but this generates spatial and temporal nonuniformities for the first row of turbine vanes. The lack of uniformity is exacerbated as combustor designs are reduced in length to reduce overall engine weight, but little is known about how the vane surface conditions are affected. This work computationally modeled the flow in a previously studied nonreacting combustor simulator, using both time-average (steady Reynolds-averaged Navier–Stokes, RANS) and time-dependent (delayed detached-eddy simulation, DDES) analyses. The effect of several dilution hole configurations on the adiabatic wall temperature and heat transfer coefficient of the first vane were studied in the time-average approach. Configurations in which the dilution jets were closer to the vane resulted in some vane surface cooling but also significant flow nonuniformity at the turbine inlet, high vane temperature gradients, and increased heat flux to the vane. Heat transfer coefficient augmentation on the vane pressure side was more than three times higher than without upstream dilution, for dilution jets close to the vane. The time-dependent DDES analysis predicted a larger degree of mixing and more uniform vane temperatures, relative to the RANS analysis, but also indicated anisotropy of the turbulence entering the turbine, which cannot be correctly captured by a RANS approach. Refinement of the time-dependent analysis in a region around the dilution jets did not significantly change the turbine inlet flowfield and had only a minor impact on the predicted vane surface temperature. Overall, the results indicate that nonuniform flow in the combustor can significantly impact vane temperature but should be modeled using scale-resolving simulations for best accuracy.

Wall thickness and injection direction effects on flat plate full-coverage film cooling arrays: Adiabatic film effectiveness and heat transfer coefficient

Adiabatic film effectiveness and heat transfer coefficient were determined for a full-coverage effusion cooled surface which simulates a portion of a gas turbine blade. Adiabatic film effectiveness and heat transfer coefficient were measured with low thermal-conductivity plastics using pressure sensitive paint and steady liquid crystal, respectively. The blowing ratio ranged from 0.5 to 2.5 with the density ratio of DR = 1.5. Geometrical parameters investigated included wall thickness (from 1.0D to 2.5D) and jet-injection directions (forward and backward injections). Local, laterally-averaged, and area-averaged adiabatic film effectiveness and heat transfer coefficients were shown to illustrate the geometrical parameter effects. Comparison of net heat flux reduction was conducted to provide an overall heat transfer estimation. Results showed that decreasing wall thickness results in opposite impacts for forward and backward injections. Also observed was that adopting backward injection for thin full-coverage effusion plate provided the highest adiabatic film effectiveness, heat transfer coefficient, and net heat flux reduction for most blowing ratios.

Length to diameter ratio effect on heat transfer performance of simple and compound angle holes in thin-wall airfoil cooling

Heat transfer coefficients on a flat plate surface downstream a row of simple and compound angle cylindrical holes are investigated using high-resolution thermographic liquid crystal technique. A variation of flow parameters including blowing ratio, and geometry parameters including compound angle and length-to-diameter ratio are examined. Blowing ratios (M) ranging from 0.3 to 2, length to diameter ratios (L/D) from 0.5 to 5, and two compound angle (β: 0°, 45°) are employed composing a test matrix of 70 test cases. Detailed local, spanwise averaged, and area averaged heat transfer coefficients hf/h0 are presented to illustrate the effect of length-to-diameter ratio and compound angle. The film cooling performance is also evaluated using NHFR method and Δφ method by combining adiabatic film effectiveness and heat transfer coefficient data. Results indicate that Δφ method has superiority in evaluating film cooling performance due to its direct reflection of temperature reduction by film protection.