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Item Thermodynamic analysis of cascade refrigeration system using R1234yf-CO2, R1234yf-R410a, and R1234yf-R134a refrigerants(Springer, 2025-05) Ranganayakulu, Chennu; Bhattacharyya, SuvanjanThe Kyoto and Montreal Protocols emphasized the necessity of replacing hydrochlorofluorocarbons and chlorofluorocarbons because of their detrimental effects on the ozone layer in the atmosphere, which shields the planet from ultraviolet radiation. Hydrofluorocarbon refrigerants, which do not disrupt the ozone layer, were inspired by all these incidents. The thermodynamic analysis of a cascade refrigeration system was examined in this work utilizing three new refrigerant pair combinations: R1234yf-CO2, R1234yf-R410, and R1234yf-R134a. Numerous operating characteristics, including compressor work, isentropic efficiency, condenser temperature, and evaporator temperature, as well as the coefficient of performance, energy efficiency ratio, and refrigerant mass flow ratio, have been examined and documented. The numerical investigation shows that with an evaporating temperature increase, there is an increase in COP, and the irreversibility of the system decreases. When there is an increase in condenser temperature, COP will decrease, and the irreversibility of the system increases. Thermodynamic analysis shows that out of three refrigerant pairs R1234yf-CO2, R1234yf-R410, and R1234yf-R134a, the COP of R1234yf-R134a refrigerant pair is 28.10% more than the refrigerant pair R1234yf-CO2 and 8.08% more than the refrigerant pair R1234yf-R410.Item Optimization of rifled tube by parametric changes using CFD for goodness factor enhancement(Wiley, 2023-11) Ranganayakulu, ChennuIn this study, extensive research was conducted using computational fluid dynamics (CFD) ANSYS®–FLUENT, with the k–ε turbulence heat transfer module to optimize the rifled-tube heat exchanger through parametric modifications for heat transfer and Goodness factor enhancement. This is based on predicting the performance of various heat transfer surfaces such as rectangular and two opposite right-angled triangular ribs. The objective of this research was to enhance the heat transfer efficiency of a rifled tube with rectangular ribs, a height of 0.775 mm, and helix angles of 30° and 58°. The research findings demonstrated that the heat transfer rate improved owing to the establishment of a secondary helical or swirl flow near the wall region, leading to an enhanced heat transfer. The modification of the rib geometry from rectangular to two opposite right-angled triangles increased the heat transfer by 17%, whereas the pressure drop remained relatively constant at 0.62 mm of rib height. This benefit is directly translated into heat transfer without any change in pressure drop. The values of the friction factor (f), Nusselt number (Nu), and goodness factor are appropriate for the new geometry in relation to the reference geometry. Rifled-tube designers can use CFD heat transfer analysis to optimize the design, reduce the need for physical prototypes, and test similar applications and rib configurationsItem Aerodynamic and Thermal Optimization of Plate Heat Exchanger Fin Arrangements(Springer, 2024-02) Ranganayakulu, ChennuPlate-fin heat exchanger systems today are exceedingly prevalent but greatly unoptimized because of the use of off-the-shelf heat exchangers, which considerably affect system efficiency. This study aims to provide a methodology for a plate-fin heat exchanger’s aerodynamic and thermal optimization to create a bespoke cooling solution based on given constraints and operating conditions. The authors studied a radiator to cool the EMRAX 208 electric motor to develop the requisite analytical and numerical models. A literature review was undertaken to compare and shortlist the best configurations, optimized through a MATLAB algorithm, and analysed and iterated in Ansys Fluent on a case-by-case basis. The results of the study point towards the superiority of serpentine fins over the other kinds, with approximately 40% higher heat transfer coefficients (at 80kmph) compared to the triangular finned models, which results in an indirect reduction in the frontal area and, subsequently, lower drag and pressure losses. The study concluded with the serpentine finned cylindrical tube heat exchanger as the best cooling solution. The exact dimensions and layout of the heat exchanger, along with the optimal fin distribution, can be obtained with the combination of the created algorithm and CFD analysis.Item Experimental investigation on condensation heat transfer coefficient and frictional pressure drop of low GWP refrigerant R1234yf on a wavy fin surface(Elsevier, 2024-01) Ranganayakulu, ChennuRefrigerant R1234yf is an ideal substitute to replace R134a due to its lower value of global warming potential (GWP). The thermophysical properties of R1234yf are similar with R134a and thus it is considered as a potential replacement for R134a in thermal fluid systems. Further, it was noticed that there is a lack of analysis on the condensation of R1234yf in a brazed plate fin compact heat exchanger (CHE) employing wavy fins. In the present work, condensation heat transfer coefficient and frictional pressure drop of refrigerant R1234yf are investigated in a brazed plate fin compact heat exchanger with wavy fin surfaces. The test condenser with wavy fin surface was selected to examine the condensation heat transfer coefficient and frictional pressure drop using refrigerants R134a and R1234yf for varying mass flux and saturation temperatures. The present study also discusses the dependency of specific kinetic energy of refrigerant on frictional pressure drop. Further, using the experimental data of wavy fin surfaces, suitable correlations for the condensation heat transfer coefficient and frictional pressure drop of R1234yf were proposed. The present paper also reports the comparison made between refrigerants R134a and R1234yf. Comparison between R1234yf and R134a shows that condensation heat transfer coefficient and frictional pressure drop of R1234yf is 9–16 % and 10–17 %, respectively, lower than of R134a in the mass flux range 15–44 kg/m2s.Item Comparative Study of Straight and Venturi Channel Cross Sections of Microchannel Heat Exchangers(ASME, 2024-09) Ranganayakulu, ChennuThis study provides extensive research on fluid flow and heat transfer for four-layered ceramic-compact counterflow microchannel heat exchangers (CFMCHE) using CFD-ACE®, a computational fluid dynamics (CFD) package. The goal is to build and expand upon previous studies in this area to identify a more efficient channel shape or cross section for better performance of the microchannel through numerical analysis under the same operating conditions. To develop the methodology for numerical analysis, a three-dimensional (3D) computational model of the CFMCHE was developed and validated with published and experimentally tested results with a percentage difference in outlet temperatures of 3–5% for hot fluids and 6–12% for cold fluids across the entire design of experiments (DoEs). Microchannel heat exchangers (MCHEs) exhibit high heat-transfer rates and area-to-volume ratios, making them suitable for industrial applications. In this study, various design options for channel cross sections in a venturi shape were assessed numerically using a validated methodology in a segmented venturi CFMCHE to enhance performance. The steady-state performance of the Venturi CFMCHE was compared to that of the straight CFMCHE baseline design under the same bucket volume, area, and operating conditions. It was found that the venturi CFMCHE showed a ∼4–9% improvement as compared to the straight CFMCHE, but same time the pumping power was also 15–40% under the same operating conditions. Making the right choice regarding feasibility often involves weighing the pros and cons. The high-power requirements are manageable in terms of the cost of high thermal performance for ground applications, such as power plants, industrial refrigeration, and air-conditioning. However, for aviation, space, and automobiles, weight/power requirements are given more weight than thermal performance. Therefore, the Venturi CFMCHE can be used for ground applications, whereas the straight CFMCHE can be used for aviation, space, and automobile applications. When the Goodness factor is plotted for all configurations for all operating conditions, it is also concluded that an improvement of ∼7.5% is observed in the two design configurations with the Venturi channel (20pc_TOP_BTM_Step and 40pc_BTM_Step) with respect to the straight channel. This implies that these two best designs can be used for all applications over the straight-channel CFMCHE.Item Numerical analysis of generation of Colburn j and friction f factor for the pin fins of a compact heat exchanger using CFD approach(Springer, 2024-03) Ranganayakulu, ChennuA Compact Heat Exchanger has a large heat transfer area per unit volume, which is achieved by utilizing extremely high-density fins. A pin fin is one of the most frequently used fins because of its advantages such as minimal pressure drop and ease of fabrication by adjusting geometric parameters such as fin height (h), fin spacing (a), fin back pitch (b), and fin diameter (d). A numerical examination was carried out on a pin fin to develop a correlation between the Colburn-j factor and friction f factor. The study was conducted for a large range of Reynolds (Re) values, encompassing both laminar (Re 200–2000) and turbulent regions (Re 2500–15000). ANSYS Fluent was used to conduct the CFD-based numerical analysis, with air at 300 K as the working fluid. The Colburn j factor and friction f factor data were acquired in this numerical study for various Reynolds numbers, as well as non-dimensional geometrical characteristics such as the fin diameter-to-fin spacing ratio (d/a), fin diameter-to-fin back pitch ratio (d/b), and fin diameter-to-fin height ratio (d/h) values. These data were validated using available experimental data from open literature. The correlations of the Colburn j factor and friction f factor were determined over a wide range of Reynolds numbers as well as the geometric characteristics of the pin fins, spanning the complete operational range of Compact Heat Exchangers for Aerospace and other applications.Item An In-Depth Comparison of Straight and Wavy Microchannel Heat Exchangers(ASME, 2024-05) Ranganayakulu, ChennuThis paper presents extensive fluid flow and Heat-Transfer studies conducted using a commercial computational fluid dynamics (CFD) package known as CFD-ACE® to elaborate and expand on the reference studies available for ceramic-compact counterflow microchannel heat exchangers (MCHEs). The computational 3D model was developed using an experimentally tested MCHE and validated with experimental data with 3–5% variation for hot fluid and 6–12% variation for cold fluid for the entire design of experiments (DoEs). This study aimed to identify the performance of novel microchannel shapes using numerical analysis. The MCHE has good heat exchange properties, a compact design at industrial throughput, and a lower inner volume. During the study and identification of novel channel shapes, the segmented wavy MCHE was evaluated. The results were compared with those of the same volume and area straight MCHE baseline design under various identical operating conditions. Although the performance in terms of effectiveness is increased up to ∼12–25% in wavy MCHE with respect to straight MCHE simultaneously, the pressure drop is also increased by ∼60–80% under the same operating conditions. Therefore, performance and trade-offs are required to make the correct decision regarding feasibility. The effectiveness of the heat-transfer enhancement was also evaluated by plotting the heat-transfer coefficient ratio with respect to the pressure ratio of the two designs under identical operating conditions. This numerical study clearly indicates that wavy channels are better from the thermal performance point of view, whereas straight channels are better from the pumping power point of view, and the quantitative values are presented in graphical form.Item Thermal performance of three-wheel and split-wheel air cycle systems for a civil aircraft environmental control system (ECS)Performance thermique des systèmes à cycle à air à trois roues et à roues séparées pour le système de contrôle de l'environnement (ECS) d'un avion civil(Elsevier, 2024-10) Ranganayakulu, ChennuThe conventional approach of regulating cabin temperature and pressure on aircraft is to utilize engine-bleed air through an environmental control system (ECS). The extraction of bleed air from the engine leads to a decrease in thrust and an increase in drag on the ECS ram air duct, resulting in higher fuel consumption. Consequently, the ECS transitioned from being engine-powered to being electrically powered. In this study, the thermodynamic characteristics of three-wheel and split-wheel air cycle systems (ACSs) with a high-pressure water separation system (HPWS) were investigated by developing a parameter decomposition model and an iterative algorithm using MATLAB for a state-of-the-art electrically driven ECS (EECS) on the Boeing 787 Dreamliner. The efficiency of the ACS was assessed by establishing analytical correlations for the coefficient of performance (COP) using relevant literature on endo-reversible thermodynamic model (ETM). By employing these analytical correlations, the thermal performance of both ACSs can be accurately predicted without the need for system modeling and simulation, considering variations in the input variables and operating conditions, such as the temperatures of fresh air and ram air, the ratio of the mass flow rates of ram air and fresh air, and component parameters, including the efficiencies of the primary and secondary heat exchangers and the pressure ratios of the fan and compressor. The implementation of a split-wheel ACS instead of three-wheel ACS in the Boeing 787 EECS led to an improvement in the COP from 0.31 to 0.43, and also resulted in a reduction of 14.35 % in the input power.Item Establishment of Colburn ‘j’ factor and Fanning friction factor ‘f’ correlations for a compact heat exchanger having perforated wavy fins using Computational Fluid Dynamics(Elsevier, 2024-09) Ranganayakulu, ChennuWavy fins find widespread application in compact heat exchangers, due to their ability to achieve high heat transfer coefficients and lower friction factors, resulting in reduced pumping power requirements and improved overall efficiency. Despite the extensive literature available on these fins, no previous study has explored the influence of perforations on wavy fins. This study aims to establish accurate correlations for evaluating the Colburn ‘j’ factor and Fanning friction factor ‘f’ in wavy fins incorporating perforations, specifically tailored for compact heat exchangers. The Colburn ‘j’ factor guides the determination of the heat transfer coefficient, while the Fanning friction factor ‘f’ is instrumental in assessing pressure drop across the fin, both essential considerations in the rating and sizing of compact heat exchangers. The numerical model involves a perforated wavy fin constructed from ‘aluminum’ with ‘air’ as the fluid medium. To derive correlations, 1458 cases were simulated using ANSYS Fluent, systematically varying geometric parameters such as fin height (h), fin spacing (s), fin thickness (t), hole diameter (d), and fin pitch (p). The study spans Reynolds numbers from laminar () to turbulent ( ). ‘j’ & ‘f’ factor values were validated using experimental data for wavy fins without perforations, followed by additional analysis incorporating perforations. The analysis showed a 19%–35% increase in the ‘j’ factor and a 21%–33% increase in the ‘f’ factor for perforated wavy fins compared to plain wavy fins. New ‘j’ and ‘f’ factor correlations were established, with over 96.3% and 95.0% of the points within ±16% and ±18% bounds, respectively. These innovative correlations applicable across the entire spectrum of operating and design conditions are expected to facilitate a more streamlined design process for compact heat exchangers featuring this unique fin geometry, enabling designers to reduce iterations during the design phase.Item Neural Network Analogy over Numerical Analysis on Thermo-Hydraulic performance factors j & f correlations development for Compact Heat Exchangers(Begell House, 2023) Ranganayakulu, ChennuA Compact Heat Exchanger (CHE) is a device used to transfer energy from one fluid to another, vital role in the efficient energy transfer. The optimum design of these Compact heat exchangers is a challenging skill for making minimum pumping power in terms of minimum pressure drop and efficient heat transfer. The generation of thermo-hydraulic performance factors, Colburn factor ‘j’ and fanning friction factor ‘f’ correlations for CHE take a large number of simulations in numerical analysis and the same is very expensive in modelling the model for Experimental analysis. In the open literature, experimentally developed fins performance as a function of Reynolds number and geometric parameters, which is expensive. The Numerical model has been developed by simulating Reynolds number Re and geometric parameters, such as fin height h, fin spacing s and fin thickness t for generation of fanning friction factor f and Colburn factor j. The 144 fin geometric parameters are used in the numerical model to develop a correlation. The numerical model is analyzed using ANSYS Fluent®. This tremendous process of correlation development is faster by using Artificial Neural Networks (ANN). This Paper focused on developing design data requirements for rectangular plain fin compact heat exchangers using CFD and Neural Networks(NN). The Analogy of correlation developed by Neural Network Prediction and CFD Fluent are verified and validated using open literature. The correlations from Neural Networks using limited data are faster and has better agreement with open literature.