Publications

2027

1. Canonical Performance Parameters for Rotating Detonation Engine Materials

   D. Vrocharis, K. B. Johnson, M. Cestari, M. Wolbrecht, B. Canales, E. Unruh, J. B. Stout, and G. Koutsakis

   In AIAA SciTech Forum, 2027

   Accepted
2. Conventional Thermal Barrier Coatings Tested Under Rotating Detonation Environments

   M. Cestari, D. Vrocharis, D. Poerschke, V. K. Champagne III, and G. Koutsakis

   In AIAA SciTech Forum, 2027

   Accepted
3. Calorimetric Heat Flux Measurements in a Hydrogen–Air Rotating Detonation Engine

   A. Samra, M. Cestari, B. Canales, M. Wolbrecht, D. Vrocharis, D. Terzis, P. Panagiotou, and G. Koutsakis

   In AIAA SciTech Forum, 2027

   Accepted
4. Semi-transparent Ceramic System Radiation Modeling Validation Against Experimental Data

   C. Pascual, and G. Koutsakis

   In AIAA SciTech Forum, 2027

   Accepted

2026

1. Forward Solution Using Fixed Backside Temperature for Inverse Heat Flux Reconstruction

   G. Koutsakis

   AIAA Journal of Thermophysics and Heat Transfer, 2026

   Accepted
2. Rational design of an architected-wall surface temperature sensor for heat flux determination

   J. B. Ghandhi, J. Carlson, R. Bonazza, D. Thompson, K. Schnittker, J. Andrews, and G. Koutsakis

   International Journal of Heat and Mass Transfer, 2026

   DOI
3. Concept of Dynamic Heat Insulation for Rotating Detonation Engines

   D. Vrocharis, and G. Koutsakis

   AIAA SciTech Forum, 2026

   Abs DOI

   This work introduces a new class of materials concept to dynamically reduce instantaneous heat fluxes in Rotating Detonation Engine (RDE) combustor chamber walls. The high-frequency and high-amplitude surface heat fluxes observed in RDEs arise from large instantaneous temperature differences between the detonation shockwave and chamber wall surface. These temperature gradients drive substantial energy loss and reduce the chamber gas pressure, ultimately limiting the cycle’s thermodynamic efficiency. This work introduces a concept for dynamically insulating the combustion chamber surfaces using surface layers or coatings with low thermal time scale. With such coatings, the surface temperature may follow the fluctuations of the cyclic detonation wave temperature, thus reducing the instantaneous heat flux and therefore cycle-mean heat flux. To analyze these cyclic thermal phenomena, a one-dimensional analytical conduction solver was utilized with the capability to handle multilayered structures. Parametric modeling was performed using transient heat flux boundary conditions representative of a hydrogen–air RDE across a broad range of coating thermal properties and engine conditions. The coating effectiveness scaled with the product of thermal time constant and detonation wave frequency and the results were non-dimensionalized to guide future materials development. This strategy may offer benefits in increasing material survivability, reducing cooling requirements, and enhancing pressure gain.

4. Inverse heat conduction in multilayer walls via partial Singular Value Decomposition

   A. Skevas, and G. Koutsakis

   International Journal of Heat and Mass Transfer, 2026

   DOI
5. Expanding the Versatility of Resonant Ultrasound Spectroscopy

   A. Garcia, K. Hanlon, M. Luci, P. R. Geimer, M. Hayne, and G. Koutsakis

   Topics in Modal Analysis & Parameter Identification in Structural Dynamics, 2026, Vol. 8, 2026

   Abs DOI Open Access

   Detection and identification of nonlinearity is a task of high importance for structural dynamics.On the one hand, identifying nonlinearity in a structure would allow one to build more accurate models of the structure.On the other hand, detecting nonlinearity in a structure, which has been designed to operate in its linear region, might indicate the existence of damage within the structure.Common damage cases which cause nonlinear behaviour are breathing cracks and points where some material may have reached its plastic region.Therefore, it is important, even for safety reasons, to detect when a structure exhibits nonlinear behaviour.In the current work, a method to detect nonlinearity is proposed, based on the distribution of the gradients of a data-driven model, which is fitted on data acquired from the structure of interest.The data-driven model selected for the current application is a neural network.The selection of such a type of model was done in order to not allow the user to decide how linear or nonlinear the model shall be, but to let the training algorithm of the neural network shape the level of nonlinearity according to the training data.The neural network is trained to predict the accelerations of the structure for a time-instant using as input accelerations of previous time-instants, i.e. one-step-ahead predictions.Afterwards, the gradients of the output of the neural network with respect to its inputs are calculated.Given that the structure is linear, the distribution of the aforementioned gradients should be unimodal and quite peaked, while in the case of a structure with nonlinearities, the distribution of the gradients shall be more spread and, potentially, multimodal.To test the above assumption, data from an experimental structure are considered.The structure is tested under different scenarios, some of which are linear and some of which are nonlinear.More specifically, the nonlinearity is introduced as a column-bumper nonlinearity, aimed at simulating the effects of a breathing crack and at different levels, i.e. different values of the initial gap between the bumper and the column.Following the proposed method, the statistics of the distributions of the gradients for the different scenarios can indeed be used to identify cases where nonlinearity is present.Moreover, via the proposed method one is able to quantify the nonlinearity by observing higher values of standard deviation of the distribution of the gradients for lower values of the initial column-bumper gap, i.e. for "more nonlinear" scenarios.

2025

1. Flow and Heat Transfer Characteristics of Additively Manufactured Mini Channels

   D. Vrocharis, and G. Koutsakis

   Volume 6: Heat Transfer: General Interest/ Additive Manufacturing Impacts on Heat Transfer; Heat Transfer: Internal Air Systems; Heat Transfer: Internal Cooling; Industrial & Cogeneration, 2025

   Abs DOI

   Abstract The development of Additive Manufacturing (AM) methods has enabled highly complex designs feasible to manufacture. This advancement has in turn rendered possible the fabrication of intricate cross sectional geometries that may be used as cooling passages for the future propulsion engines. These internal cooling channels may have rougher surfaces and smaller hydraulic diameters, approaching mini-channel dimensions around 1 mm, compared to conventional designs. The internal AM channel characteristics complicate the determination of the friction factor and heat convection coefficient, as they deviate from existing correlations. In this work, an AM mini-channel with relative surface roughness ε/DH = 0.025 and a rectangular hydraulic diameter of DH = 1.78 mm is experimentally tested under variable mass flow (10–35 · 10−3 kg /s) and heat flux (100–250 kW/m2) conditions. For the data demonstration the j factor of the Chilton-Colburn modified analogy is used, to combine data of different studies. The friction factor demonstrated transition to fully turbulent flow at Re ≈ 104 whereas for higher values of Re, the data showed satisfying agreement with values predicted by the Colebrook equation. The convective heat transfer coefficient correlated to the Gnielinski correlation for the range of Re number applied, but the data trendline showed a steeper slope in the j-Re diagram. This behavior could be an indication of the problem being dependent to additional parameters other than the well-established Re, Pr and f numbers.

2024

1. Optical absorption effects in thermal radiation barrier coating materials

   G. Koutsakis, and D. R. Clarke

   Surface and Coatings Technology, 2024

   DOI Open Access
2. Design Metrics for Highly Reflective Fibrous Insulation

   G. Koutsakis, N. Deliwala, and D. R. Clarke

   Advanced Materials Technologies, 2024

   Abs DOI

   Abstract The high melting temperatures, large optical bandgaps, and large refractive indices of oxides favor their use as fibrous insulation at extreme temperatures where radiative heat transfer is significant. In this work, the reflectance of mats of fibers is modeled with the Monte Carlo (MC) method using the fiber optical properties (such as the scattering efficiency and scattering coefficient), the fiber radius, refractive index, volume fraction ( f v ), as well as the thickness of insulation ( L ). Two key metrics are identified for the design of highly reflective insulation. One is a novel metric, the Kuhn scattering length, which is based on an analogy between the anisotropic random walks in MC radiative transfer and polymer physics. It is used to determine approximate the effective thickness of insulation ( f v L ). The second is a size metric, which indicates that fiber mats are most reflective when the fiber radius is . The model is validated on experimental measurements of commercially available insulation consisting of fibers with 3–5 µm radius.

2022

1. Fracture-based transient thermo-mechanical analysis of reciprocating engine thermal barrier coatings

   G. Koutsakis, M. R. Begley, J. W. Hutchinson, and J. B. Ghandhi

   Engineering Fracture Mechanics, 2022

   DOI
2. Analytical solution of unsteady heat conduction in multilayer internal combustion engine walls

   G. Koutsakis, and J. B. Ghandhi

   Applied Thermal Engineering, 2022

   DOI
3. Experimental and Analytical Study of Temperature Swing Piston Coatings in a Medium-Duty Diesel Engine

   A. Babu, G. Koutsakis, S. L. Kokjohn, and M. J. Andrie

   SAE International Journal of Advances and Current Practices in Mobility, 2022

   Abs DOI

   The use of Thermal Barrier Coatings (TBCs) has been shown to be a promising technology to improve internal combustion engine efficiencies by reducing heat rejection to the coolant and oil. In recent studies, temperature swing coatings that have simultaneously low volumetric heat capacity and low thermal conductivity have been shown to be particularly promising in this regard. In this study, a traditional and a newer swing coating are applied to the piston of an on-road medium-duty diesel engine to assess the benefits of their use. An analytical wall temperature model is coupled to the 1-D engine simulation software GT-POWER and predictions of wall temperature, heat transfer and chemical heat release rate are presented. The swing coating is found to yield an  1.2% efficiency benefit at the highest load condition studied alongside an 80°C improvement in exhaust temperature at the lowest load condition studied compared to a reference uncoated piston. Further, the traditional TBC was not found to have a significant efficiency advantage over the reference uncoated piston, highlighting the promise of the swing coating in improving the thermal efficiency of on-road diesel engines.

4. Optimization of thermal barrier coating performance and durability over a drive cycle

   G. Koutsakis, and J. B. Ghandhi

   International Journal of Engine Research, 2022

   Abs DOI

   A methodology to thermo-mechanically optimize a piston thermal barrier coating over a full drive cycle was established. The optimization objective was to minimize the heat transfer to the engine wall while maintaining structural integrity of the coating. Over 800 candidate materials were investigated and the optimization required more than one million non-road transient drive cycle calculations; real materials were investigated to ensure a realizable result and the existence of thermal and mechanical properties. High computational efficiency was achieved using a recently developed analytical heat transfer technique for multilayer engine walls. An uncoupled approach was utilized for the optimization, wherein the gas temperature and heat transfer coefficient profiles from a fully coupled and calibrated baseline model over the 20-min drive cycle were employed. The coating/piston interface temperature was constrained to be below the maximum piston service temperature limit. The durability was assessed using a recently developed analytical coating delamination framework for engine in-cylinder coatings based on the energy release rate when a crack forms. Results are presented for a mechanically unconstrained optimization and for cases constrained to three fixed levels of drive-cycle maximum energy release rate, and also constrained by the individual material’s toughness. The best-performing coating materials identified were verified using the fully coupled system-level model, which compared well to the uncoupled predictions. A study on the effect of adding a sealing layer to some high-performing, but porous, coatings showed a reduction in fuel consumption benefit and an increased exhaust temperature over the cycle, but the system still outperformed the uncoated case. The results of the study elucidate the importance of including engine performance and mechanical failure considerations in thermal barrier coating design.

5. Delamination Failure on High-Output Diesel Engine Thermal Barrier Coatings

   G. Koutsakis, J. C. Saputo, E. Gingrich, M. J. Tess, S. Sampath, and J. B. Ghandhi

   SAE technical papers on CD-ROM/SAE technical paper series, 2022

   Abs DOI

   An analytical mechanics model was employed to predict the delamination of several thermal-barrier-coated pistons that had been previously tested in a high-output, single-cylinder diesel engine. Some of the coatings delaminated during engine operation. Results are presented for two thicknesses of the same coating material, and for two similar coatings with different levels of stiffness. All the coating thermomechanical properties such as thermal conductivity, density, volumetric heat capacity, thickness, elastic modulus, coefficient of thermal expansion, Poisson ratio and toughness, were measured prior to engine testing. Previous measurements of the piston transient heat flux, based on fast-response surface temperature data, in the same engine were used as an input to calculate the multilayer wall temperature distribution. A theoretical methodology was employed to evaluate and predict the coating durability. The method considers the release rate of elastic strain energy stored in the coating when it cracks as the main driving force for delamination; when the calculated energy release rate overcomes the toughness, coating crack failure is predicted to occur. The mechanics durability model was found to provide good trendwise comparison with the post-run coating integrity observations. When the energy release rate significantly exceeded the toughness, failure was indeed observed. Borderline cases, where the energy release rate was close to the material toughness, however, were more difficult to predict.

2021

1. Assessment of In-Cylinder Thermal Barrier Coatings over a Full Vehicle Drive Cycle

   G. Koutsakis, S. Miles, and J. B. Ghandhi

   SAE technical papers on CD-ROM/SAE technical paper series, 2021

   Abs DOI

   In-cylinder thermal barrier coatings (TBCs) have the capability to reduce fuel consumption by reducing wall heat transfer and to increase exhaust enthalpy. Low thermal conductivity, low volumetric heat capacity thermal barrier coatings tend to reduce the gas-wall temperature difference, the driving potential for heat transfer from the gas to the combustion chamber surfaces. This paper presents a coupling between an analytical methodology for multi-layer coated wall surface temperature prediction with a fully calibrated production model in a commercial system-level simulation software package (GT-Power). The wall surface temperature at each time step was calculated efficiently by convolving the engine wall response function with the time-varying surface boundary condition, i. e., in-cylinder heat flux and coolant temperature. This tool allows the wall to be treated either as spatially uniform with one set of properties, or with independent head/piston/liner components. Steady state results are presented for five modern, engine-specific coating architectures selected from the literature. The coating performance, on a system-level basis, was found to be engine-condition dependent. The optimum material for one performance parameter may not provide any benefit to another. To comprehensively compare TBCs relative to the uncoated baseline, a full transient drive cycle simulation was performed for two coatings using the 20-minute certification Non-Road Transient Cycle (NRTC). Experimental boundary conditions along with ECU data were provided from production engine test cell data. A reduction of in-cylinder heat transfer and fuel consumption was found for both coatings, while exhaust enthalpy was increased by 0.5% in spite of the fuel mass saving. For a piston-coated scenario, a maximum of 1.5% reduction in fuel consumption and consequently a similar level of brake specific CO2 reduction was realized over the drive cycle, depending on the coating architecture.

2. Comment on “Numerical approach to define a thermodynamically equivalent material for the conjugate heat transfer simulation of very thin coating layers” by P. Olmeda, X. Margot, P. Quintero, J. Escalona, International Journal of Heat and Mass Transfer, Vol. 162 (2020) 120377

   J. B. Ghandhi, and G. Koutsakis

   International Journal of Heat and Mass Transfer, 2021

   DOI

2020

1. Surface temperature of a multi-layer thermal barrier coated wall subject to an unsteady heat flux

   G. Koutsakis, G. Nellis, and J. B. Ghandhi

   International Journal of Heat and Mass Transfer, 2020

   DOI
2. An Analytical Approach for Calculating Instantaneous Multilayer-Coated Wall Surface Temperature in an Engine

   G. Koutsakis, and J. B. Ghandhi

   SAE International Journal of Advances and Current Practices in Mobility, 2020

   Abs DOI

   Thermal swing coatings that have low volumetric heat capacity and low thermal conductivity are attractive because they have the potential to significantly reduce heat transfer to the combustion chamber walls. This paper presents an analytical method for determining the exact solution of the time-resolved wall temperature during the engine cycle for any number of coating layers and properties using the Laplace transformed heat diffusion equation. The method relies only on material properties and the past heat flux history, and represents the exact solution of the heat diffusion equation. The analytical nature of the solution enables fast computation and, therefore, application to system-level optimization calculations. The model relies on an assumption of one-dimensional heat flow, and constant material properties. The major advantage of this approach compared to the standard finite difference approach, wherein the wall is finely discretized, is that there is no approximation and the accuracy is guaranteed, i.e., it does not depend on nodal density. Results are presented for both a quasi-steady operating condition and for an engine transient event. In the latter case a 200-fold reduction in computation time relative to a finite difference code was realized.