Kothadia, Hardikkumar Bhupendra

This study investigates subcooled flow boiling at subatmospheric system pressure for high heat flux transfer applications where higher surface temperatures are a concern. The experimental setup includes an SS-304 tube with a 11.7 mm diameter, a 1500 mm heated length, and 0.5 mm thickness. The mass flux ranges from 90 to 300 kg/m2s with a varying heat flux of 30-145 kW/m2. Results indicate that the subcooled flow boiling heat transfer coefficient is enhanced at lower subatmospheric system pressure due to more nucleation sites activated by higher wall superheat. The surface temperature is radially symmetric at subatmospheric system pressure, and the onset of nucleation is achieved earlier in the axial direction with higher heat flux. The mass flux does not significantly affect the subcooled flow boiling heat transfer coefficient. These findings may be useful in designing subcooled flow boiling systems for various applications.

Latent thermal energy storage devices can efficiently store surplus thermal energy during off-peak hours. The system's phase change material (PCM) improves energy storage capacity and isothermal properties. Most PCMs have weak thermal conductivity, which hampers heat transfer. Thus, the present analysis involves an insight into the heat transfer characteristics and thermal performance of a vertical tube-in-tank cold storage system. The objective of this investigation is to gain a better understanding of the significance of buoyancy-driven convection within the side bulk region during the discharging of water as PCM in the heat exchanger. A series of experiments are conducted to investigate the effect of varying the initial bulk temperature on the rate of PCM solidification. The effects of three distinct initial bulk temperatures which are 20 °C, 15 °C, and 5 °C on the solidified mass fraction, thermal performance, and heat transfer rate at various radial and axial locations in PCM are examined. It is seen that PCM experienced a varying cooling rate with varying axial height and is found to be the highest in the bottom region. PCM temperature decreases from top to bottom under the cases of 20 °C and 15 °C, respectively. In contrast, a narrow-ranged thermocline layer is observed under 5 °C bulk temperature, making uniform temperature distribution within the bulk. The influence of bulk natural convection prevails in the later stages of the discharging process under 5 °C bulk, whereas it predominantly exists during the initial stages of solidification under bulk temperatures of 20 °C and 15 °C.

The present study explores the effect of a sweeping jet on heat transfer over a flat surface. A primary incompressible jet is directed through a nozzle and is given a cyclic sweeping effect by injecting a secondary jet periodically. The primary jet deflects to either of the walls as a result of the cyclic perturbation caused by the Coanda effect. A 2D numerical study has been conducted on a vertical surface placed at a distance of 6d from the nozzle. A constant heat flux condition is applied on the flat surface of 600 × 300 × 0.1 mm3. The plate is mounted vertically, and the oscillating jet is impinged normal to the surface. The thermal and flow characteristics near the surface of the plate are computed using SST k-ω turbulence model. A study is performed at different oscillation frequencies and jet-blowing ratio to investigate the heat transfer characteristics. Results indicated that compared to a steady jet, the oscillating jet greatly enhances heat transmission. The results show the isotherm, velocity contour, Nusselt number distribution, and vortex formation. The local heat transfer was found to be significantly influenced by the oscillating jet. The optimal frequency for the various circumstances was evaluated, with higher frequencies producing significant augmentation of heat transmission. These results indicate that the use of an oscillating jet can enhance heat transfer performance, and the frequency of oscillation in industrial applications where high thermal efficiency is required for heat-exchanging systems should be carefully considered. Furthermore, an ensemble study is conducted to investigate the stagnation Nusselt number behavior and blowing ratio along with the effect of vortex bubbles on the cooling of the heated plate.

In this experimental study, a comprehensive analysis of the flash evaporation of a static liquid pool is conducted. The study investigates the impact of different parameters on the process, including the initial temperature of the water pool ranging from 50 to 85 ◦C and primary pressure between 5 to 50 kPa (abs.) with corresponding superheat levels ranging from 7 to 38 ◦C. The research highlights that superheat is the primary driving force behind flash evaporation operation, and its impact reduces over time. Initially, a high superheat value is present, which affects the temperature drop and evaporated mass evolution throughout the process, along with the initial water pool temperature.

Pool boiling is cost-effective, simple and prevalent amongst all available cooling schemes. The lack of quantitative data, qualitative theories and explanations in the area of critical heat flux (CHF) in mini tubes makes it an interesting domain of research. This paper discusses the effect of heater length, diameter, orientation and subcooling on CHF. There is the scarcity of literature that considers all the effects altogether. This paper analyses the effect of all the parameters together in a single study. SS304 mini tubes of 2.5- and 1.2-mm diameters with various lengths are used as a heater in horizontal and vertical orientations. The water pool is kept at 30, 50, 75 °C and saturation temperature. The study is performed at atmospheric pressure. It has been observed that with the increase in pool temperature, the magnitude of CHF decreases. The shortest length has a higher magnitude of CHF corresponding to a given pool temperature. The values of CHF decrease with an increase in tube diameter. The CHF value is higher for horizontal as compared to vertical orientation. An empirical correlation, including the effect of subcooling, orientation and tube dimensions is derived from the CHF data. The experimental data is presented in a tabulated form to contribute to pool boiling CHF databank.

Strengthening the existing CO2 capture technologies is crucial for averting the imminent climate crisis. The present study undertakes the algae-assisted microbial fuel cell (MFC) for indirect CO2 capture via bicarbonate utilization through natural photosynthesis process. The flue gas is first cooled using a heat exchanger and then directed to a sieve-plate absorption column where sodium carbonate supplemented wastewater absorbs CO2 generating flue gas-derived bicarbonates (FGDBs). The FGDBs are added in the plastic bag photobioreactors (PBRs) coupled with the MFC for absorption by Chlorella vulgaris. Adding FGDB at the MFC cathode increased the algae biomass productivity two times (0.677 ± 0.086 Kg/m3/d) compared to the cathode without FGDB. The algae could efficiently utilize 76.84 ± 1.23 %. More than 50 % of sodium carbonate can be recycled for the next round of CO2 capture. FGDB supplementation at the cathode improved MFC's electrical energy production (0.0066 kWh/m3) by 1.5 times through enhanced anodic and cathodic currents. Therefore, the present study offers a biochemical CO2 sequestration process that generates power, algae biomass, and treats water by utilizing algae-assisted MFC for flue gas carbon capture.

There is colossal research scope in achieving greater functionality and durability in microelectronic devices as conventional air cooling systems cannot handle these devices' cooling requirements which may be due to their low heat transfer performance. In such instances, phase change heat transfer mechanisms are appropriate for heat extraction since they utilise both sensible and latent heat. The ability to remove huge amount of heat at low wall superheats and the lack of moving parts make pool boiling appealing. This experimental study emphasises the analysis of bubble behaviour and critical heat flux (CHF) on electrodeposited tubes developed via in-house built set-up. Copper has been deposited over SS 304 tubes of a length of 100 mm and diameter of 2.5–4.5 mm. Their surface morphologies have been investigated using various characterization techniques such as SEM, EDS, AFM, and XRD. The surface roughness measurement confirms that electrodeposition increases the roughness of the tube. The contact angle analysis confirms that the coated surfaces are hydrophobic in nature. The CHF visualization is carried out under uniform heat flux conditions. It is observed that with an increase in diameter, the magnitude of CHF reduces irrespective of the coated and uncoated surface. The hydrophobic surface exhibits a lesser CHF magnitude than the uncoated surface. The reduction in CHF magnitude is within a range of 4.27%–8.93%. At lower heat flux magnitude, the bubble nucleation is rigorous in coated tubes, making it efficient for phase change heat transfer applications.

Ray tracing technique has been efficiently used in the design of optical fields associated with concentrated solar thermal technologies (CSTT). To estimate the available energy for a given solar field, designing an optimized solar field layout is a preliminary requirement. For a given solar field, the effective transfer of concentrated solar energy to its working fluid via. receiver plays a critical role. Thus, the optimized optical layout for the solar field along with designing a receiver is an essential need to obtain overall efficiency of the systems/sub-systems associated with CSTT. In this proposed work, we have used heliostats having dimensions of 5 m x 5 m (area = 25 m2) and 2 m x 2 m (area = 4 m2), and the heliostats were laid out in a circular manner. The proposed (considered) receiver with curved geometry having suitable dimensions provides maximum thermal energy efficiency for an optical radiation impinging on the receiver. By estimating the amount of flux reaching the receiver, we estimate the total optical energy reaching the receiver for a given heliostat field layout in a day. This investigation will help in developing and designing of solar fields with better efficiencies. The proposed methods allow us to investigate the uniformity of illumination of rays and its distribution on the receiver (curved geometry tube as receiver).

An experimental study has been implemented to examine water pool sudden flash evaporation in a cuboidal chamber. Some key parameters have been studied such as initial vacuum tank pressure, superheat, and initial water temperature. The initial water volume is set to one liter only. Initial conditions of the liquid vary from 50 °C to 85 °C temperature and vacuum tank pressure 6.33 kPa and 20.63 kPa (abs.) which corresponds to 2 °C to 38 °C superheat. Moreover, the influence of these key parameters on the thermo- properties like water pool temperature, mass evaporated, nonequilibrium fraction (NEF), and heat transfer coefficient (h) are analyzed. The experimental results showed that lower vacuum tank pressure and greater superheat have larger impact on flashing phenomenon. Results suggested that a larger value of the NEF and h take place at the beginning of the flash. The value of heat transfer coefficient found to be time dependent function tends to decrease with flash time passes.

Rapid cooling can be obtained by a very innovative technique of depressurizing liquid in a controlled volume. This feature of attaining the possible minimum temperature within a few minutes led this technology to be used for several essential applications such as waste-heat recovery, desalination, drying of nuclear waste, and thermal management of aerospace equipment. The present study intends to develop a flashing experimental system to obtain insights into the mechanisms and the influential parameters to establish a controllable flash technology. The operating parameters such as the initial temperature of water present in a flash chamber and depressurized liquid through a connected vacuum tank are shown to play a vital role in controlling the flashing conditions. A rigorous number of experiments were performed ranging from 45° C to 85° C with varying pressurizing effects to obtain the optimum flashing conditions as per the applications. The drop in the water temperature due to the sudden pressure drop created inside the flash chamber connected with a large vacuum tank is measured by connected K-type thermocouples. A comparison of flash evaporation with normal evaporation/cooling is also presented, which confirmed the high efficiency of 90% of cooling obtained through flash evaporation.