Kothadia, Hardikkumar Bhupendra

The transfer of higher heat flux at subatmospheric system pressure is relevant to different applications where the higher surface temperature is a constraint. There is a scarcity of literature on subcooled flow boiling at subatmospheric system pressure. The experiments are performed at 50, 75 and 101 kPa absolute system pressure in an SS-304 tube of 5.85 mm radius, heated length of 1500 mm and 0.5 mm thickness. The mass flux of 90–300 kg/m2s is streamed at the varying heat flux of 48–218 kW/m2. The heat transfer coefficient in single-phase flow is not impacted significantly by the subatmospheric system pressure. The subcooled flow boiling heat transfer coefficient is increased at the lower subatmospheric system pressure. The more nucleation sites due to higher wall superheat are activated at the higher heat flux at subatmospheric system pressure. The surface temperature is radially symmetric at subatmospheric system pressure for all values of the Froude number. The gravitational force is suppressed at subatmospheric system pressure. The onset of nucleation is achieved earlier in the axial direction at the higher heat flux. The mass flux has no significant effect on the subcooled flow boiling heat transfer coefficient. The pressure drop and fluctuations are enhanced at the higher mass flux, higher heat flux and lower subatmospheric system pressure.

Droplet impingement is one of the phase change techniques used for equipment cooling because of the proper utilization of sensible and latent heat. This study analyzes droplet evaporation on aluminum and copper substrate. The instantaneous heat transfer coefficient (HTC) during evaporation of 100 µl de-ionized water is studied. The infrared camera is used to visualize and analyze the evaporation mechanism. The effect of substrate and fluid temperature on droplet evaporation is analyzed. The surface temperatures of both substrates are varied from 105 °C to 165 °C. The test fluid is maintained at temperatures of 30 °C, 50 °C, 75 °C, and 99.4 °C. The droplets exhibited Leidenfrost behavior for temperatures beyond 145 °C with copper and 150 °C with aluminum surface. It is observed that the evaporation rate is higher for copper than aluminum. A user-friendly interface is developed for determining the instantaneous surface area of an evaporating droplet. The apparent peak in the area in copper is higher than aluminum for all cases due to surface tension gradient and Marangoni flow. HTC increases with the temperature of droplet evaporating on a given substrate and is higher for copper. When substrate temperature increases for a given droplet temperature, the instantaneous HTC increases.

Vortex generator can be used to enhance heat transfer as it is capable of providing better mixing. Vortex generator is easy to manufacture and install in heat transfer units without much additional cost. Present work is planned to investigate the effect of vortex generators on local heat transfer coefficient, critical heat flux (CHF) and pressure drop to ensure design and safety of heat transfer units with vortex generators. Experiments are carried out for three values of pitch 22, 32 and 41 mm and an angle of attack of 45° of vortex generators with horizontal configuration in a smooth tube of diameter 11.9 mm, length 1,000 mm and 0.4 mm wall thickness with R-123 as the working fluid. The effect of mass flux (175–681 kg/m2s) and heat flux (7.9 − 221.7 kW/m2) are studied. Both horizontal and vertical-oriented configurations of the vortex generator are tested. Local wall temperature measurement is carried out with Infrared thermography. Comparing results with plain tube shows the heat transfer coefficient and CHF enhancement. Horizontal orientation (HO) of the vortex generator gives better results than the vertical orientation (VO). Heat transfer coefficient, CHF and pressure drop penalty increase with a decrease in the pitch of the vortex generator.

This paper proposes a fast cooling technology called flash evaporation or flashing to overcome cooling problems. Flashing is a pressure-driven phase change phenomenon widely studied because of its complex physics and numerous applications. An experimental system is fabricated to study the efficiency of the proposed process by considering water as a working fluid. A transparent acrylic column is used as the flash chamber, enabled with a high-speed camera to visualize and capture this fast and complex process. Experiments have been carried out with various initial temperatures from 65 °C to 80 °C, initial vacuum tank pressure ranging from 11.32 kPa to 41.32 kPa, and initial water heights of 80 mm and 140 mm. The flashing phenomenon is divided into two stages based on its effectiveness. From the present study, it becomes apparent that the drop in the temperature increases with the initial temperature and decreases with increasing vacuum tank pressure. The cooling rate is improved by 49.1% by increasing the initial pool temperature by 23%. The present study demonstrates an effective application of the flashing process in areas requiring high cooling rates and rapid vapour generation, such as refrigeration and drying.

Phase change material (PCM) based energy storage systems are a promising solution to ensure a continuous energy supply from intermittent renewable sources for long-term applications. This study explores the potential of economical, industrial-grade sodium acetate trihydrate (SAT) for thermal energy storage, as pure SAT is expensive. Unlike conventional approaches to mitigate supercooling, this research induces controlled supercooling for long-term usage of latent heat activation. Composites, including Tween 80, coconut oil, and ethylene glycol, were incorporated into industrial-grade SAT to analyze their effects on supercooling behavior. The morphological and thermophysical properties of composite PCMs (CPCMs) were analyzed. PCM samples with 30g were heated to 65 °C, 80 °C, and 95 °C, then cooled naturally, with crystallization triggered using a heterogeneous seeding technique. The influence of composites at varying concentrations, with mass ratios of 1/2 and 1/3, was evaluated for their effects on the crystallization temperature and supercooling degree. Additionally, the behavior of copper particles submerged in supercooled SAT and air-surface interactions was examined. The experimental results revealed that Tween 80-based CPCMs exhibited the highest degree of supercooling across all conditions, while coconut oil-based CPCMs showed an interesting trend at higher initial temperatures of 95 °C, where the degree of supercooling increased, a behavior not observed at lower temperatures. Conversely, ethylene glycol-based CPCMs exhibited poor crystallization kinetics, tailoring the maximum heat release temperature from 58 °C to 40 °C, which makes them suitable for specific thermal applications. In all CPCMs, higher composite concentrations increased supercooling, highlighting the need to optimize composite levels for desired thermal performance. © 2025 Elsevier Ltd

The work described in this manuscript involves systematic analysis of four types of basic and advanced thermodynamic cycles for two-bed continuous adsorption cooling system with activated carbon-ammonia as the working pair. The four cycles under consideration are Basic cycle, Mass recovery cycle (MRC), Heat recovery cycle (HRC), Combined heat and mass recovery cycle (CHMRC). Using ammonia as the refrigerant allow us to build a pressurized system, and greater level of compactness of the system might be achieved as compared to water, ethanol and methanol-based adsorption cooling systems operating under vacuum condition. The usage of ammonia as refrigerant also allows the recovery of high-grade waste heat from exhaust gases coming out of large automobiles having temperature range of 250–500 °C. Three most important performance parameters associated with adsorption cooling, namely: COP, SCE and second law efficiency are evaluated and compared against five control parameters, namely: maximum desorption temperature, minimum adsorption temperature, condensation temperature, evaporation temperature and heat capacity ratio between bed construction material and adsorbent. A simple yet novel iterative scheme is proposed and developed in detail to estimate the equilibrium pressure and associated thermal and compositional states of the two beds after the completion of highly irreversible mass recovery process. © 2025 Elsevier Ltd

This study addresses the critical prediction of frictional pressure drop in two-phase flow at subatmospheric system pressures, essential for improving heat transfer methodologies in various industrial applications to enhance thermal equipment design efficiency. The pressure drop in the heat transfer equipment is affected by the system pressure, system geometry and working fluid. There is a dearth of literature on two-phase pressure drop in the conventional adiabatic tubes at subatmospheric system pressures. Experimental investigations are conducted using adiabatic tubes of 8, 13.7, and 18 mm diameters and 1500 mm length at 0.25, 0.5, 0.75, and 1 bar system pressures to get two-phase frictional pressure drop. The steam at 0 – 0.98 vapor qualities is used at 32 – 660 kg/m2s mass flux in the adiabatic tubes. The effects of tube diameter, vapor quality, mass flux and system pressure on two-phase frictional pressure drop are investigated. In the liquid–vapor flow, the pressure drop experiences a non-linear increase with changes in vapor quality. The frictional pressure drop in two-phase flow is elevated with higher vapor quality, reduced subatmospheric system pressure, increased mass flux, and a smaller tube diameter. Reducing system pressure from atmospheric to 0.25 bar doubles the frictional pressure drop in an 18 mm tube and increases it by 12 kPa in a 13.7 mm tube at high mass fluxes (≥ 56 kg/m2s) and vapor qualities (≥ 0.2). A correlation is suggested to predict the two-phase pressure drop with reasonable accuracy in the adiabatic tubes at subatmospheric system pressures.

The study focused on examining the area-averaged heat transfer characteristics of an air jet impinging on a concave surface. The experiments involved varying the Reynolds number (Re) within the range of 10,000 to 75,000, while adjusting the jet-to-plate distance from 3 to 20 times the jet diameter (d). The experimental setup included a jet with a diameter of 11.5 mm, and concave surfaces with curvature radii of 218 mm (Curvature ratio (Cr) = 0.026) and 281 mm (Cr = 0.021) were used. A detailed investigation was conducted using Infrared Thermography to measure the average Heat Transfer Coefficient (HTC) of jet impingement on these concave surfaces. The aim was to assess the impact of surface curvature on heat transfer at different locations from the impingement point. Additionally, the research compared the heat transfer effects in circular and square impingement regions that were identical in radial span and area ratios. It was observed that the area-averaged Nusselt number (Nu) generally increased with a higher Re and a lower jet-to-plate distance ratio (z/d). A maximum average Nu increase is observed for Cr = 0.026 when the mass flow rate is from 10,000 to 75,000. Increasing the curvature is an effective approach to provide localized cooling. A correlation comparison was also conducted, and a new empirical correlation has been proposed. This correlation works effectively across various ranges of Re, z/d, Cr, and radial position (r/d) with a deviation of less than ± 11%. Graphical Abstract: (Figure presented.)

Highly concentrated graphene oxide-based phase change material (PCM) composite is investigated for its heat transport behaviour when applied in a cold storage system. Optimizing thermal properties and behaviors during phase change is crucial to mitigate the bottlenecks of poor thermal conductivity resulting in the thermal resistance of a proper and faster heat transfer during solidification. The current study considers an efficient way to enhance the bottlenecks by synthesizing a highly concentrated GO aqueous solution. Differential scanning calorimetry and scanning electron microscopy are conducted to characterize the thermal and morphological properties of the composite matrix while to address the stability issue associated with solid-liquid change, variation of rheological properties for differently concentrated GO-PCM composites with temperature and shear rate is conducted. A non-Newtonian nature is obtained with lower shear rates, while GO exhibits a Newtonian behaviour under higher shear rates. Viscosities for highly concentrated PCM composites are found to be temperature-sensitive. In addition, dependency upon the crucial operating factor during the phase change process is discussed. Operating factors like heat transfer fluid (HTF) inlet temperature and its mass flow rate are investigated during the time-wise temperature evolution in the PCM composite. Inlet HTF temperature is found as a key parameter to shorten the process by 33 % compared to the mere effect obtained while increasing the mass flow rate. The addition of GO particles is found to be beneficial after the latent heat gets released, which helps shorten the total solidification time by 42 %. However, a decrease in the total storage capacity is found when compared with DI water.

Channels of different sizes are employed in diverse engineering applications to enhance the efficiency of heat transfer system. Minichannels are preferred for their compactness, enhanced heat transfer, larger surface area to volume ratio, and high heat transfer coefficient values. There is a dearth of literature addressing flow boiling in minichannels with highly subcooled water at atmospheric system pressure. This research paper analyses the flow boiling phenomenon in a 2 mm diameter minichannel, comparing it with conventional 4 mm and 11.7 mm diameter tubes while maintaining a constant length to diameter ratio of 150. The analysis investigates the wall temperature distribution, local heat transfer coefficient, and temporal heat transfer fluctuations at a mass flux of 150 – 2400 kg/m2s and 109 – 1080 kW/m2 heat flux. The study explores the impact of boiling number on wall temperature fluctuations. Results reveal that mass flux doesn't impact heat transfer in subcooled flow boiling but does in saturated flow boiling in conventional channel. There are no significant temporal fluctuations of wall temperature in a 4 mm tube. However, vapor bubble size affects the circumferential area of minichannels, leading to abrupt flow disturbance. Wall temperature fluctuations occur in the minichannel for the boiling number exceeding 1.76 × 10−4. Temperature fluctuations in minichannel increase with boiling number, but decrease as mass flux is increased. Delving into the depths of minichannel heat transfer, this intriguing study unveils the pivotal role of the boiling number in ensuring process stability.