Raj Kumar Dahal

Turbomachinery flows are widely known to be naturally unsteady, mainly due to the relative motion of rotors and stators. The stator/rotor interaction, especially, is a three-dimensional (3D) unsteady phenomenon driven by wake, shock, and blade interactions that can lead to significant periodic unsteadiness, reaching up to 50% of the mean flow values. These interaction effects, combined with inlet-temperature distortions (hot streaks) caused by non-uniformities in the combustor-exit flow temperature originating from discrete fuel-injector configurations upstream, significantly increase flow complexity. In high-pressure (HP) turbines subjected to inlet temperature distortions, strong secondary flows and the non-uniform redistribution of hot fluid across stationary and rotating rows dictate the localized aerothermal behaviour. While direct multipassage unsteady simulations that capture the full annulus provide accurate representations, they remain computationally prohibitive for practical design cycles—particularly in turbine stages with unequal blade counts. To mitigate this computational burden, this paper evaluates a single-passage numerical framework utilizing phase-shifted periodic boundary conditions to capture deterministic unsteadiness frequencies without geometric modifications. Firstly, a comprehensive grid independence study is performed using steady-state formulations; secondly, the single-passage phase-lag solver is rigorously validated against a multi-passage reference simulation; and finally, the single-passage framework is utilized to systematically investigate the aerothermal impacts of three distinct radial hot-streak alignments (hub-, mid-, and tip-aligned profiles) under adiabatic and non-adiabatic wall constraints. The results reveal critical physical insights into hot-streak migration paths, preferential blade heating, and the strong coupling between fluid thermal fields and core-stage aerodynamics.

Turbomachinery flows are widely known to be naturally unsteady, mainly due to the relative motion of rotors and stators. The stator/rotor interaction, especially, is a three-dimensional (3D) unsteady phenomenon driven by wake, shock, and blade interactions that can lead to significant periodic unsteadiness, reaching up to 50% of the mean flow values. These interaction effects, combined with inlet-temperature distortions (hot streaks) caused by non-uniformities in the combustor-exit flow temperature originating from discrete fuel-injector configurations upstream, significantly increase flow complexity. In high-pressure (HP) turbines subjected to inlet temperature distortions, strong secondary flows and the non-uniform redistribution of hot fluid across stationary and rotating rows dictate the localized aerothermal behaviour. While direct multipassage unsteady simulations that capture the full annulus provide accurate representations, they remain computationally prohibitive for practical design cycles—particularly in turbine stages with unequal blade counts. To mitigate this computational burden, this paper evaluates a single-passage numerical framework utilizing phase-shifted periodic boundary conditions to capture deterministic unsteadiness frequencies without geometric modifications. Firstly, a comprehensive grid independence study is performed using steady-state formulations; secondly, the single-passage phase-lag solver is rigorously validated against a multi-passage reference simulation; and finally, the single-passage framework is utilized to systematically investigate the aerothermal impacts of three distinct radial hot-streak alignments (hub-, mid-, and tip-aligned profiles) under adiabatic and non-adiabatic wall constraints. The results reveal critical physical insights into hot-streak migration paths, preferential blade heating, and the strong coupling between fluid thermal fields and core-stage aerodynamics.

This study presents a surrogate-based aerothermal optimization of a High-Pressure Turbine (HPT) stage, subject to realistic nonuniformities in combustor-exit temperature and velocity. Specifically, for a combustor-to -nozzle-guide-vane (NGV) count ratio of 1:2, the two NGVs are shaped independently to accommodate inlet flow non-uniformity. A surrogate model based on steady-flow CFD samples is constructed and coupled with a Genetic Algorithm (GA) to optimize NGV lean parameters under coupled thermal and swirl inlet non-uniformities. The objective function was formulated based on the total-temperature distribution at the NGV exit, which was identified as the primary driver of rotor tip (RT) heat flux. The optimized unequal vane pairing resulted in a significantly cooler heat flux pattern across the RT and casing region. This avoided costly, unsteady CFD runs by training the surrogate on steady RANS samples. This thermal improvement is attributed to a favorable redistribution of the NGV exit temperature and a modified streamline topology of the rotor blade (pressure side). Furthermore, the optimized geometry yielded an increase in overall stage aerothermal efficiency of 1.7%.

Natural fiber-reinforced composites perform poorly when exposed to moisture. Biocarbon has been proven to improve the water-absorbing behavior of natural fiber composites. However, the interaction effect of the design parameters on the biocarbon-filled hemp fiber-reinforced bio-epoxy composites has not been studied. In this study, the effects of the design parameters (pyrolysis temperature, biocarbon particle size, and filler loading) on the water absorptivity and water diffusivity of hemp-reinforced biopolymer composites have been investigated. Biocarbon from the pyrolysis of hemp and switchgrass was produced at 450, 550, and 650 °C. Composite samples with 10 wt.%, 15 wt.%, and 20 wt.% of biocarbon fillers of sizes below 50, 75, and 100 microns were used. The hemp fiber in polymer composites showed a significant influence in its water uptake behavior with the value of water absorptivity 2.41 × 10⁻⁶ g/m².s^1/2. The incorporation of biocarbon fillers in the hemp biopolymer composites reduces the average water absorptivity by 44.17% and diffusivity by 42.02%. At the optimized conditions, the value of water absorptivity with hemp biocarbon and switchgrass biocarbon fillers was found to be 0.72 × 10⁻⁶ g/m².s^1/2 and 0.73 × 10⁻⁶ g/m².s^1/2, respectively. The biocarbon at 650 °C showed the least composite thickness swelling due to its higher porosity and lower surface area. Biocarbon-filled hemp composites showed higher flexural strength and energy at the break due to the enhanced mechanical interlocking between the filler particles and the matrix materials. Smaller filler particle size lowered the composite’s water diffusivity, whereas the larger particle size of the biocarbon fillers in composites minimizes the water absorption. Additionally, higher filler loading results in weaker composite tensile energy at the break due to the filler agglomeration, reduced polymer-filler interactions, reduced polymer chain mobility, and inadequate dispersion of the filler.