Reverse Flow Combustor Research Articles

Ignition and Lean Blowout Characteristics of a Reverse-Flow Combustor for an Ultra-Compact Gas Turbine Engine

Ignition and Lean Blowout Characteristics of a Reverse-Flow Combustor for an Ultra-Compact Gas Turbine Engine

Experimental and numerical characterization of hydrogen combustion in a reverse-flow micro gas turbine combustor

This study conducts experimental and numerical investigations on a stagnation-point, reverse-flow combustor to examine the effect of its design features on the combustion behavior and emission characteristics of non-premixed, turbulent hydrogen/air flames. The study covers a wide range of equivalence ratios, from ultra-lean to stoichiometric, and thermal power inputs varying between 5 kW and 20 kW. The main motivation behind this design is to achieve passive recirculation of flue gas within the combustor to enhance dilution of the incoming charge, thus potentially leading to relatively uniform thermal fields, lower temperatures, and suppressed NOx emissions. This design concept is implemented in a micro gas turbine – a practical energy conversion device tapped to play a major role in decarbonizing CHP systems suitable for small-scale industries. Optical chemiluminescence measurements, in addition to thermocouple-based temperature and Quantum Cascade Laser (QCL) spectroscopy-based exhaust emissions measurements, were conducted for all the test cases. Spatial OH* signal intensity is used as heat release signature to identify distinct features of the test flames, allowing for useful information on the nature and dynamics of the combustion process within the combustor. The results show stable hydrogen/air flames for all the operating conditions; however, a more distributed combustion is observed at higher powers for all the given equivalence ratios. Relatively higher NOx emissions at φ >0.2 suggest the lower achieved mixing between burnt and fresh gases, particularly in the non-preheated case, prevent a thorough dilution of the fresh mixture. The latter is necessary to achieve low-temperature, uniform thermal fields – an essential for low thermal NOx emissions in gas turbine combustors. Further, CFD results show that preheating considerably changes flow field dynamics and recirculation pattern for better mixing; however, this also dramatically increases thermal NOx emission, with the lowest levels observed at φ = 0.2, while the peak occurs at φ = 0.4. Finally, a ‘brute-force’ sensitivity analysis shows that thermal NO and NNH pathways contribute the most to NO formation in hydrogen/air flames.

Effect of primary hole position and ignition position on the ignition performance of a reverse flow combustor

The purpose of this paper is to analyze the influence of the primary hole position and ignition position on the flow field and ignition performance of the reverse flow combustor. Igniters of two schemes are located on the side wall, and the other two are in the outer liner. The lower primary hole with the axial distance value of 29.2 and 24.7 mm are employed for each type of scheme. These four schemes are experimentally studied by particle image velocimeter (PIV) and high-speed camera. The differences of ignition performance between the four schemes are analyzed from the flow field structure and ignition process. The results show that the propagation mode of flame kernel will change with the change of ignition position. When the flame kernel forms in the central region of the combustor, both upper and lower recirculation vortices are ignited simultaneously, resulting in a shorter ignition time. However, when the flame kernel is formed near the outer liner, the upper recirculation vortex ignites first, and then the lower recirculation vortex is ignited, resulting in a relatively longer ignition time. Ignition performance is mainly determined by the flow velocity near the flame kernel. There is a negative correlation between ignition performance and the local flow velocity. Increasing the transverse flow velocity near the flame kernel, the ignition performance will decrease. The overall ignition time is significantly influenced by the propagation path of the flame kernel. In a reverse flow combustor, changes in the position of the primary hole result in two typical flow fields, the ignition time mainly depends on the distance between the flame kernel and the recirculation vortices.

Investigation of a low emission peripheral vortex reverse flow (PVRF) combustor fuelled by LPG and ethylene

Peripheral Vortex Reverse Flow (PVRF) combustor is characterized by a strong peripheral recirculation zone resulting in stable combustion and low pollutant emissions. In this study, a PVRF combustor is investigated in the premixed and non-premixed modes with LPG and ethylene fuels at a maximum heat load of 6.25 kW and thermal intensity of 25 MW/m3-atm, relevant to gas turbine combustors. The fuel is injected co-axially, with respect to the air jet, using two different fuel injection diameters of 1 mm, and 2 mm. For both the fuels, the fuel injection velocity is higher as compared to the air injection velocity for fuel injection diameter of 1 mm, while it is lower for fuel injection diameter of 2 mm. CH* chemiluminescence images are acquired, and CO and NOx emissions are measured. Reynolds Averaged Navier Stokes (RANS) simulations are carried out at an equivalence ratio of 0.8 using the Eddy Dissipation Concept (EDC) model with a H2/CO/C1–C4 mechanism comprising of 50 species and 373 reactions. Delayed ignition and downstream shifting of the reaction zone facilitating higher product gas entrainment prior to reactions is observed in the non-premixed mode as compared to the premixed mode. NOx emissions were found to be lower in the non-premixed mode as compared to the premixed mode for both the fuels. This is possibly due to suppression of NO formation in the reaction zone via. prompt route which is also supported by lower CH* chemiluminescence intensity in the non-premixed mode. With LPG, the case with larger fuel injection diameter showed further delayed mixing as compared to the case with smaller fuel injection diameter, and resulted in even lower NOx emissions. For all the cases, CO emissions were of similar levels in the non-premixed and premixed modes suggesting that lower NOx emissions in the non-premixed mode is not at the expense of increase in CO emissions. Overall, the investigation shows that lower NOx emissions can be achieved in the non-premixed mode as compared to the premixed mode if more product gas entrainment is facilitated prior to the reactions by delayed mixing between fuel and oxidizer.

Experimental Investigation on Flame Chemiluminescence and Flow Characteristics of a Reverse-Flow Combustor

The reverse-flow combustor is extensively applied in small engines due to its compact structure. The cold-state flow characteristics, the ignition process, and flame chemiluminescence characteristics of the reserve-flow combustor were investigated experimentally. Using the particle imaging velocimeter, the cold-state time-averaged flow fields at three different total pressure loss coefficients ζ B are examined. The ignition processes of the combustor, as well as flame characteristics during stable combustion at different equivalent ratios φ , are analyzed based on flame chemiluminescence imaging when ζ B is 3%. As ζ B increases from 1% to 5%, the flow field of the combustor remains almost unchanged while the flow velocity increases. For ζ B =3%, the ignition time T drops from 26 ms to 16 ms with an increase of φ . In addition, the ignition performance is enhanced. Proportions of the T2 phase (quasi-stable phase) are 73%, 68%, and 66%, respectively, suggesting that the quasi-stable phase is the key to successful ignition. As φ increases, the flame becomes thinner and shorter. Furthermore, standard deviations of continuous image areas of CH ∗ and OH ∗ drop from 0.0143 and 0.0132 to 0.0115 and 0.0109, respectively, which indicates the enhancement of combustion stability. According to the brightness distributions of CH ∗ and OH ∗ along the axial direction of the combustor at different equivalent ratios, the production of OH ∗ is significantly affected by the temperature. It can be adopted as an important sign of heat release during combustion.

Nonlinear Dynamic Analysis of the Transition from MILD Regime to Thermoacoustic Instability in a Reverse Flow Combustor

ABSTRACT In the present study, the stability limit of the MILD regime of combustion has been studied in a reverse flow combustor using syngas (20% H2, 20% CO, 60% N2) as fuel. Pressure oscillations obtained from the combustor are analyzed using regular approaches from the dynamical systems theory as the oxidizer preheat temperature varies from 772 K to 424 K. Stable MILD combustion is obtained at preheat temperatures ≥724 K; at lower preheat temperatures combustion becomes unstable, leading to high amplitude periodic oscillations. Permutation spectrum test and recurrence plot of the MILD condition unravels the deterministic nature of the pressure oscillations. The correlation dimension confirms that the system has high-dimensional chaotic characteristics. At temperatures <724 K, the correlation dimension becomes close to 1 as the system transitions to combustion instability. In these conditions, the permutation spectrum test shows close to zero standard deviation in all ordinal indices, the phase-space shows doughnut-shaped structures, and recurrence plot consists of long diagonal lines, all confirming the periodic nature of the system. The dominating frequency during combustion instability is attributed to the second axisymmetric mode of the combustor. Finally, three recurrence quantification criteria, namely, Recurrence Rate, Determinism, and Laminarity, and K-value obtained from 0 to 1 test, are tested as indicators for the onset of combustion instability. Determinism and K-value are recommended for capturing the transition in the underlying dynamics.

Experimental Investigation on Flow Characteristics of a Reverse-Flow Combustor

Reverse-flow combustor is widely used for small engines to overcome high speed shaft whirling problem and to provide a low frontal area. An experimental investigation was carried out to research the flow field characteristics of a reverse-flow combustor in this paper. Different aerodynamic conditions were studied using PIV to reveal the characteristics of both the nonreacting and reacting flow fields. The structure of the nonreacting flow field in the central section shows similarity as the total pressure loss coefficient increases. The penetrating depth, jet angle, recirculation zone position, and the flow streamlines are similar, while the velocity value of the flow field increases. The structure of the reacting flow field on the central section is different from that of nonreacting flow field, but the variation trend of the reacting flow field under different pressure loss coefficient is similar to that of the nonreacting flow field. By examining the nonreacting and reacting flow fields under the same total pressure loss conditions, marked differences were observed in the primary zone close to the swirler outlet. The relative motion between fuel injection, airflow, and combustion affects the flow field in this zone. The velocity with combustion is faster than that of the nonreacting flow because of the increased temperature and heat release.

Non premixed operation strategies for a low emission syngas fuelled reverse flow combustor

In the present study, we investigate the combustion of low-calorific value syngas in a reverse-flow combustor at a fuel thermal input of 3.3 kW. A reverse-flow combustor is known to improve stability while reducing pollutant emissions due to internal recirculation. The study is significant for renewable energy applications of syngas obtained from biomass gasification. Equivalence ratio and O2% in the oxidizer have been varied to establish different modes of operation at atmospheric pressure (P = 1.013 bar). Temperature, heat release, CO, and NOx emissions have been measured to characterize the operation modes. The heat release varies significantly with O2% and influences the CO emissions. The NOx emission is insensitive to the parameters due to the low-calorific value of syngas which limits the maximum temperature to around 1700 K. The lowest CO emission of 32 ppm and NOx < 1 ppm has been achieved in the present study. The combustion efficiency of the combustor is greater than 99%. The variations in pollutant formation were explained using reactor network models. The instantaneous images of heat release revealed key features of the operation modes. As the N2 dilution level is increased, the chemical timescale increases, leading to a volumetric heat release. Suppression of localized heat release reduces the maximum temperature and consequently the NOx emissions.

Influence of structure on the combustion characteristics of a small aero-gas turbine engine combustor

In this paper, a combustor using vortex plates instead of swirler as the head swirl device has been studied. The flow and combustion characteristics in the combustor mainly depend on the combustor structural characteristics, therefore, the study of combustor structural parameters on combustion characteristics is a key means for the improvement of combustor performance. Numerical simulation method was used to study the flow and combustion characteristics in the primary combustion zone of a small aero-gas turbine engine annular reverse-flow combustor. The results show that the nozzle mounted on the flame tube outer wall is better than on the head considering the benefit to fuel/air mixing and combustion performance for this type combustor. The cooperation between vortex plates and primary combustion holes is the method of organizing flow in the primary combustion zone, and the cooperation is effective than simply relying on the vortex plates or the primary combustion holes. The circumferential flame propagation mainly relies on the vortex plates according the results. The number of vortex plates has effect on the shape of the combustor head reverse-flow zone, and there is an optimal value to optimize the combustion performance.

Experimental investigation of a Jet-A1 fuelled peripheral vortex reverse flow combustor

Combustion characteristics of a novel peripheral vortex reverse flow (PVRF) combustor are investigated using Jet-A1 fuel. Previously, we have demonstrated the operation of PVRF combustor using LPG and ethanol fuels, and here we demonstrate and investigate the operation of PVRF combustor using Jet-A1 fuel. The combustor was operated at atmospheric pressure with preheated inlet air (300 °C) and high thermal intensity (25 MW/m3-atm at ø = 0.8) relevant to gas turbine combustors. Conventional swirler was not employed and the reactants were injected as jets. Combustion was primarily stabilized due to a dominant peripheral vortex of hot burned gases inside the combustor. In the non-premixed mode, the liquid fuel was injected coaxially w.r.t. the incoming air-jet and it was atomized due to strong shearing from the annular air jet. Shadowgraphy was used to investigate the atomization characteristics of the fuel. Electrochemical gas analyser was used to measure exhaust emissions (NOx, CO). The combustor demonstrated low emissions and stable operation over a wide range of equivalence ratios. CH* chemiluminescence was used to study the location and the dynamic behaviour of the reaction zone. The reaction zone was positioned further downstream in the non-premixed mode as compared to the prevaporized-premixed mode suggesting that atomization, evaporation and mixing of fuel in the non-premixed mode is causing a delay for the onset of the reactions. From shadowgraphy images, it was observed that retraction of the fuel jet considerably improved the fuel atomization.

Flame development characteristics inside a reverse-flow combustor at early stages of combustion

Characteristics of flame development and the influence of reverse-flow configuration on these characteristics few seconds after ignition are investigated experimentally. Simultaneous and high-speed OH∗ chemiluminescence and pressure measurements are performed. Methane is used as the fuel, and the fuel–air equivalence ratio is 0.7. Four experimental conditions pertaining to mean bulk flow velocities of 4.0 and 6.4 m/s along with two igniter rod positions (−5 and 1.5 mm with respect to a flame-holder) are tested. The results suggest the flame development can be categorized into ignition, stabilization, and transition phases. During the ignition phase, compared to closed ducts, the reverse-flow configuration allows for achieving relatively large values of the normalized flame edge velocity prior to reaching the stagnation wall. Once the flame reaches the wall, the pressure rate significantly increases. After this, the flame stabilizes on the flame-holder forming a Bunsen-type flame, which is followed by a long term oscillation in the heat release rate for the majority of the tested conditions. During this period, the pressure, the pressure rate, and the spatially-averaged heat release rate feature a peak at the acoustic frequency of the combustor. The normalized power spectrum densities of the pressure and spatially-averaged heat release rate collapse and follow power-law relations, agreeing with the results reported in the literature pertaining to unconfined flames. This suggests, despite significant influence of the stagnation wall during the ignition and stabilization phases, spectral characteristics of pressure and heat release rate are not influenced by the presence of the wall during the transition phase.

Performance and Emissions of Drop-In Aviation Biofuels in a Lab-Scale Gas Turbine Combustor

Abstract There is a growing need for drop-in biofuels for gas turbines for enhanced energy security and sustainability. Several fuels are currently being developed and tested to reduce dependency on fossil fuels while maintaining performance, particularly in the aviation industry. The transition from traditional fossil fuels to sustainable biofuels is much desired for reducing the rapidly rising CO2 levels in the environment. This requires biofuels to be drop-in ready, where there are no adverse effects on performance and emissions upon replacement. In this study, the performance and emissions of four different aviation drop-in biofuels were evaluated. They include UOP HEFA-SPK, Gevo ATJ, Amyris Farnesane, and SB-JP-8. These aviation biofuels are currently being produced and tested to be ready for full or partial drop-in fuels as the replacement of traditional jet fuels. The characteristic performance of each fuel from the prevaporized liquid fuels was performed in a high-intensity (20 MW/m3-atm) reverse flow combustor. The NO emissions showed near unity ppm levels for each of the fuels examined with a minimum at an equivalence ratio of ∼0.6, while CO levels were in the range of 1000–1300 ppm depending on the fuel at an equivalence ratio between 0.75 and 0.8. For an equivalence ratio range between 0.4 and 0.6, NO and CO emissions remained very low (between 1–2 ppm NO and 2400–2900 ppm CO) depending on the fuel. The examined biofuels did not show any instability over a wide range of equivalence ratios from lean to near stoichiometric condition. These results provide promising results on the behavior of these drop-in aviation biofuels for use in high-intensity gas turbine combustors providing stability and cleaner performance without any modification to the combustor design.

Investigation of novel scaling criteria on a reverse-flow combustor

Abstract The present study investigates the scaling of a 3.3-kW laboratory scale combustor (baseline) to 25-kW using Detached Eddy Simulation (DES) with detailed chemistry. The combustor operates on syngas under atmospheric conditions. Two novel scaling criteria, namely, constant volume to jet momentum ratio (CM) and constant volume to jet kinetic energy ratio (CK) have been introduced and applied to the 3.3-kW combustor. Additionally, the constant velocity (CV) and constant residence time (CRT) scaling criteria have also been investigated for comparison. Flow, mixing, temperature, and their effect on the reaction zone and emissions have been compared for the scaled-up combustors with the laboratory scale combustor. The scaling criteria were observed to significantly influence the location and volumetric distribution of the reaction zone. In terms of performance parameters, the CV criterion entails the lowest pressure drop and CO emissions. The performance of the CK and CM criteria closely follows that of the CV criterion, thus offering a promising scaling methodology for reverse-flow combustors. The chamber scaled by the CRT criterion recorded the highest pressure drop and CO emissions. The insights from the present study will aid in sizing lab-scale combustors to industrial requirements.

Performance of a novel mixed-flow trapped vortex combustor for turboshaft engine

This paper describes a novel compact combustor that utilizes trapped vortex in a single cavity to provide flame stability. This novel combustor combines the characteristics of the reverse-flow combustor and the straight-through combustor, named mixed-flow trapped vortex combustor (TVC). All the liquid RP-3 kerosene is injected directly into the cavity. Combustion performances such as pressure drop, lean blowout (LBO) limits, temperature profiles, combustion efficiency and emissions were examined experimentally using a sector test rig under ambient pressure with a temperature of 300, 433 and 500 K, respectively. Experiment results showed that the pressure loss varied as the square of the inlet velocity and not exceeded 5%. The overall fuel to air ratio (FAR) of LBO limits was less than 0.0043, and the lowest LBO limits was achieved at the lowest inlet air velocity and highest temperature. The effect of cavity equivalent ratio on the temperature profiles was greater than the inlet air velocity. There was an optimal cavity equivalent ratio for a given inlet air velocity to make the temperature profile closer to the desired temperature profile. Combustion efficiency was above 96% under all operating conditions. The lowest combustion efficiency occurred at the highest inlet velocity. An increase in the inlet velocity results in an increase of the concentration of unburned hydrocarbons (UHC) and carbon monoxide (CO). The emission index of oxides of nitrogen (EINOx) was less than 2.1 under all operating conditions.

Mesoscale combustor-powered thermoelectric generator: Experimental optimization and evaluation metrics

The recent development of potable power sources is of considerable interest. A mesoscale combustor-powered thermoelectric generator integrated with a mesoscale stagnation-point reverse-flow combustor and a built-in heat exchanger was designed and optimized in this work to augment electric power and overall efficiency. The mesoscale combustor can run at a volumetric heat load of 72 MW/m3. The thickness of heat spreader, input power, equivalent ratio, fuel types, and heat exchanger channel types were explored to optimize the system performance. The optimized electric power of 30.7 W obtained at an overall efficiency of 3.21% was larger than the previous record of 18.1 W at an overall efficiency of 3.01% in 2015. The proposed thermoelectric generator, which was conducted with 13.2 kg of methane, reached an energy density of 300 Wh/kg. The novel design provided a concrete way to solve the contradiction between the availability of Bi2Te3-based thermoelectric module and its relatively low working temperature. Additionally, a new metrics, which is defined as (EFS/efTE,max) at Pin, was firstly proposed to evaluate various thermoelectric generators, in which different thermoelectric materials, combustion types, and input powers could be involved. The proposed metrics correlates the overall, running thermoelectric, maximum thermoelectric, combustion (reaction), and heat collection efficiencies under different input powers. The power of this metrics was comprehensively discussed by deducing the running thermoelectric efficiencies of various previous works and presenting the performance comparisons of different previous thermoelectric generators. The obtained metrics, that is, (75.9%/4.23%) at 957 W in this work was higher than those in previous studies.

Combustion Characteristics of a Peripheral Vortex Reverse Flow Combustor With Coaxial Fuel Injection

Abstract This paper deals with an experimental investigation of a novel and simple reverse flow combustor, operated stably with a liquid fuel (ethanol) for heat release intensities ranging from 16 to 25 MW/(m3·atm) with very low NOx and CO emissions. The liquid fuel is injected coaxially with the air jet along the centerline of the combustor. The high velocity air annulus helps in primary breakup of the liquid fuel jet. Air injection along the combustor centerline results in a strong peripheral vortex inside the combustor leading to enhanced product gas recirculation, internal preheating of the reactants, and stabilization of reaction zones. Single-digit NOx emissions were obtained for both coaxial fuel injection (non-premixed) and a premixed–prevaporized (PP) cases for all operating conditions. CO emissions for both the modes were less than 100 ppm (ϕ &amp;lt; 0.75). CH* chemiluminescence images revealed two distinct flame structures for coaxial fuel injection case. A single flame structure for PP case was observed extending from the injector exit to the bottom of the combustor. The instantaneous (spatially averaged) CH* intensity fluctuations were significantly lower for the PP case as compared to the coaxial fuel injection case.

Numerical Investigation of NOx Emission Reduction in Non-Premixed Lean Reverse-Flow Combustor in a Micro Gas Turbine Engine

This paper details a NOx emissions characteristics study of a reverse-flow combustor using the steady Reynolds averaged Navier-Stokes (RANS) methodology. The combustor considered in this paper is the SR-30 engine model. The aim of this work is to understand the pollutant formation characteristics within the combustor chamber volume across various combustor zones. Effective ways to counter such high emissions by means of flow dilution were also investigated in the present study. Results indicated that the baseline reverse-low combustor model showed high NOx emission characteristics in the primary and secondary zones due to uneven temperature distribution. The emissions can be reduced effectively with additional dilution holes at the dilution zone of the reverse-flow combustor. The flow split due to the newly designed holes caused the velocity components (axial, radial and tangential), turbulence kinetic energy and the temperature within the primary and secondary zone to reduce significantly. The recovery of flow in the dilution zone with further reduction in temperature reduced the overall NOx emission significantly.

Investigation of peripheral vortex reverse flow (PVRF) combustor for gas turbine engines

In a reverse flow combustor air is injected from the exit side so that the bulk gas flow reverses its direction while exiting the combustor. In the combustor investigated here, a dominant peripheral vortex away from the exit is stabilized by carefully choosing the location of the air jet. Combustion is stabilized due to internally recirculated hot product gases in this peripheral vortex. The fuel is injected in the peripheral vortex region at high velocity and rapid mixing of injected fuel with the oxidizer results in avoidance of hot spot regions and leads to overall lean combustion and low pollutant emissions. Due to reverse flow geometry, along with the presence of the peripheral vortex, favourable residence time distribution of gases is achieved (shown numerically). This resulted in lower CO emissions (confirmed experimentally). In the non-premixed mode, fuel injection port is either placed on the exit/air port side (named as RS configuration) or on the opposite side of the exit/air port (named as RO configuration). The combustor operating in RO configuration displayed wider lean operational limit, and lower CO emissions as compared to the RS configuration, while NOx emissions were almost same in both the cases.

Investigation of the Effect of Perforated Sheath on Thermal-Flow Characteristics Over a Gas Turbine Reverse-Flow Combustor—Part 2: Computational Analysis

Abstract The objective of Part 2 is to employ a computational scheme to investigate the difference in flow pattern, pressure drop, and heat transfer in a gas turbine’s dump diffuser and over the outer surface of the combustor with and without a sheath. Both experimental and computational studies are performed. In Part 1, the experiments were conducted under low pressure and temperature laboratory conditions to provide a database to validate the computational model, which was then used to simulate the thermal-flow field surrounding the combustor and transition piece under elevated gas turbine operating conditions. For laboratory conditions, the computational fluid dynamics (CFD) results show that (a) the predicted local static pressure values are higher than the experimental data but the prediction of the global total pressure losses matches the experimental data very well; (b) the total pressure losses are within 3% of the experimental values; and (c) the temperature difference between the sheathed and non-sheathed cases is in the range of 5–10 K or 16–32% based on the temperature scale between the highest and lowest temperatures in the computational domain. Under the elevated pressure and temperature conditions in real gas turbine, removing the sheath can achieve a significant pressure recovery of approximately 3% of the total pressure, but it will be subject to a wall temperature increase of about 500 K (900 °F or a 36% increase) on the outer radial part of the transition piece, where the flow is slow due to diffusion and recirculation in the large dump diffuser cavity near the turbine end. If modern advanced materials or coatings could sustain a wall temperature of about 500 K higher than those currently available, the sheath could be removed. Otherwise, removal of the sheath is not recommended.

Investigation of the Effect of Perforated Sheath on Thermal-Flow Characteristics Over a Gas Turbine Reverse-Flow Combustor—Part 1: Experiment

Abstract Reverse-flow combustors have been used in heavy, land-based gas turbines for many decades. A sheath is typically installed over the external walls of the combustor and transition piece to provide enhanced cooling through hundreds of small impinging cooling jets, followed by a strong forced convection channel flow. However, this cooling is at the expense of a large pressure loss. With the modern advancements in metallurgy and thermal-barrier coating technologies, it may become possible to remove this sheath to recover the pressure loss without causing thermal damage to the combustor chamber and the transition piece walls. However, without the sheath, the flow inside the dump diffuser may exert nonuniformly reduced cooling on the combustion chamber and transition piece walls. The objective of this paper is to investigate the difference in flow pattern, pressure drop, and heat transfer distribution in the dump diffuser and over the outer surface of the combustor with and without a sheath. Both experimental and computational studies are performed and presented in Part 1 and Part 2, respectively. The experiments are conducted under low pressure and temperature laboratory conditions to provide a database to validate the computational model, which is then used to simulate the thermal-flow field surrounding the combustor and transition piece under elevated gas turbine operating conditions. The experimental results show that the pressure loss between the dump diffuser inlet and exit is 1.15% of the total inlet pressure for the non-sheathed case and 1.9% for the sheathed case. This gives a 0.75 percentage point (or 40%) reduction in pressure losses. When the sheath is removed in the laboratory, the maximum increase of surface temperature is about 35%, with an average increase of 13–22% based on the temperature scale of 23 K, which is the difference between the bulk inlet and the outlet temperatures.