Research

Sponsored Projects
Research Areas
Research Facilities


Sponsored Projects

Permanent Dropwise Condensation via Amphiphilic Additives in Vapor Phase Psychrometry Driven Design and Fabrication of An All-Season Optimal Atmospheric Water Harvester Passive Two-Phase Heat Spreader for Hotspot Mitigation in Microgravity of Space
  • Funding Agency: HSFC ISRO
  • Status: Ongoing (2020-2022)
  • Amount: 24 Lakhs
Development of an Ionic Liquid-based Ultra-High Heat Dissipation Module for Energy Efficient Boiling Systems
  • Funding Agency: CRG, SERB
  • Status: Ongoing (2020-2023)
  • Amount: 47 Lakhs
Surface Active Additives for Enhanced Flow Boiling in Microchannel Development of an Agricultural Waste Based Off-the-grid Cimate Control Unit for Storage and Processing of Agricultural Produces
Strengthening Interfacial Characterization Facilities: Funds for Improvement of S&T Infrastructure
  • PI: HOD, Department of Mechanical Engineering
  • Co-PI: Dr. Rishi Raj is one among the 6 co-PIs
  • Funding Agency: DST FIST
  • Status: Ongoing (2019-2024)
  • Amount: 297 Lakhs
Acoustic Detection of Leidenfrost Dynamics on Scalable Micro-/Nanostructured Surfaces
  • Funding Agency: DST Nanomission
  • Status: Completed (July 2016- July 2019)
  • Amount: 27 Lakhs
Design and Development of an Agricultural Waste Based Gasifier Heating System for GreenCHILL
  • Co-PI: Dr. Ajay. D. Thakur, Department of Physics
  • Funding Agency: MHRD and DST under UAY
  • Industry Partner: New Leaf Dynamic Technologies
  • Status: Completed (August 2016- August 2018)
  • Amount:Amount: 95 Lakhs
Enhancement of Boiling Heat Transfer via the Suppression of Coalescence in Microgravity
  • Funding Agency: RESPOND ISRO
  • Status: Completed (April 2015- April 2018)
  • Amount: 27 Lakhs
Flow Boiling Heat Transfer in Scalable Nanostructured Microchannels for High Heat Flux Applications
  • Co-PI: Dr. S. K. Saha, IIT Bombay
  • Funding Agency: DST SERB
  • Status: Completed (August 2014- August 2018)
  • Amount: 50 Lakhs


Research Areas

TFTL focuses on the investigation of thermal and fluid transport during phase change for a variety of energy, water, and thermal management applications, both for ground and space-based applications. The research performed at TFTL can be broadly classified into three interrelated areas listed below:

[A] Boiling Heat Transfer for Thermal Management Applications


Thermal management is critical for safe and reliable operations of high power density applications such as high-density electronic chips, solar cells, power generation plants (thermal and nuclear), space, and defense systems, among others. Efficient heat removal via boiling which utilizes latent heat of vaporization has the potential to address thermal management issues. TFTL focuses on understanding the key aspects of boiling to devise various state-of-the-art cooling strategies, devices, and feedback control systems for boiling applications.

Boiling Acoustics and Deep Learning
Advantage of utilizing the latent heat of vaporization stored in liquids makes boiling a popular choice of heat transfer in various applications such as desalination, power-generation, refrigeration and air conditioning, and electronic thermal management among others. Central to the boiling process is the bubble ebullition cycle (nucleation, growth, departure, and rewetting), which drives the heat transfer from the heated surface. Ebullition cycle gets interrupted at critical heat flux (CHF), wherein vapor generation rate is significantly high to break the contact between the heater and the replenishing liquid. CHF, which is also referred to as boiling crisis, is the maximum heat flux a surface can accommodate in high heat transfer coefficient (HTC) nucleate boiling regime before transition to low-HTC film boiling regime. A heat flux value slightly above CHF results into a drastic and uncontrollable temperature surge, sufficient to cause catastrophic failure (burnout). Further, the large uncertainties associated with CHF prediction values (±50%) are compensated for by using large factor of safety in practical applications. Inability to reduce the margin to safety has forced the community to instead look for mean to enhance CHF to address the future increase in heat dissipation requirements. Development of prognostic/diagnostic tools for boiling is required to reduce the margin to CHF and thus enable the boiling-based systems operate at near-CHF heat flux values. However, boiling is highly sensitive to a large number of experimental parameters such as the heater surface superheat, wettability, roughness, orientation, dissolved gas concentration, the level of subcooling in the boiling fluid, heating pattern, and system operating pressure, among many others. Hence, the development of prognostic/diagnostic tools is long-awaited.

We at TFTL take this challenge at hand very sincerely and explore the acoustic emissions (AEs) during boiling to further the prognostic/diagnostic tool development [A1, A4, A8]. Our study demonstrates the promise of the frequency signature of the AE as a powerful tool to detect various features in boiling systems including the important events such as the onset of nucleate boiling, the critical heat flux, and the transition boiling regime. We therein report a unique feature in the frequency signature of the acoustic emissions, wherein the peak frequency range shifts from 200 Hz-250 Hz in nucleate boiling regime to 400 Hz-500 Hz at CHF. We demonstrated that acoustic-based feedback control can detect the CHF and turn down the power to control thermal runaway.

We further use the AE spectrograms of various boiling regimes to train a convolutional neural network, which shows a validation accuracy of 99.92% against the ground truth [A1]. Despite the variations in boiling surfaces, working fluids, and the heating strategy between the training and the evaluation datasets, the network accurately predicts the respective boiling regimes. We use the insights to perform advance prediction of boiling crisis for mitigating thermal runaway-induced accidents in boiling-based systems. Currently, we are working in a direction for the development of an acoustic and deep learning-based tool which is efficient, adaptive, and supports Internet of Things (IoT).



Pool Boiling Heat Transfer: Role of Additives
Boiling heat transfer has an order of magnitude higher heat transfer coefficient (HTC) in comparison to single-phase heat transfer. With the ever-increasing demand for high heat flux dissipation within a smaller temperature budget, investigation of HTC enhancement techniques is of significant importance. At TFTL, we focus on passive techniques such as cooling fluid modification via the addition of surface-active-agents. We have recently demonstrated that HTC increases up to 30-80 percent in comparison to baseline case of water [A7, A12, A13]. Such smaller superheat implies lesser irreversibility and hence can enhance the system efficiency in addition to improving reliability for temperature-related failures. Further, we also developed a mechanistic model to predict the critical heat flux (CHF) with these surface-active-agents based on vapor crowding [A7, A13]. The understanding gained from these investigations helped us in unifying the CHF dependence on foamability of various surface-active-agents [A2, A6]. Currently, we are investigating the role of various surface-active-agents for simultaneous enhancement of HTC and CHF.

Thermal Management Solutions for Space Applications
Boiling based passive cooling strategies do not work as effectively in space as on the earth due to weaker/absence of buoyancy. We recently demonstrated a robust passive technique which uses surfactants found in common soaps and detergents to avoid coalescence and remove bubbles downwards, away from an inverted heater [A15]. Bubble removal frequencies in excess of ten Hz resulted in more than twofold enhancement in heat transfer in comparison to pure water. Currently, we are investigating ionic liquids as additives in boiling fluid to further enhance heat transfer performance [A15]. Preliminary results demonstrate 2.4 times and 4.5 times enhancement in heat transfer coefficient (HTC) and critical heat flux (CHF), respectively, in comparison to water. We are also working to design and develop additive based wickless heat spreaders for hotspot mitigation form earth and space based electronics [A3].

Flow Boiling in Microchannels
Miniaturization and advancement of electronic devices demand high heat flux dissipation in excess of 100 W/cm2. In this regard, flow boiling based heat sinks are envisioned as a viable solution to dissipate high heat flux within the minimal temperature. However, thermo-hydraulic fluctuations in these heat sinks is a bottleneck. We at TFTL have devise novel techniques to suppress thermo-hydraulic fluctuations during flow boiling in microchannels. We grow nanostructures [A11] on the walls of the microchannel to maintain thin-film thereby suppressing the fluctuations. Further, we have recently demonstrated that modification in the manifold design can effectively be used to suppress temperature and pressure fluctuations up to a very high heat flux [A10]. Currently, we are working on strategies such as adaptive vapor venting to completely mitigate thermo-hydraulic fluctuations throughout boiling regimes [A5].

[B] Colloids and Interfacial Science


Solid-liquid interactions in the presence of a pervading medium such as vapor or another liquid give rise to a number of phenomena around us. Common examples include morning dew, rain, fog, and foam, which are marked by the formation of distinct interfaces between the participating phases. Understanding the physics of interfaces is central to manipulating the properties of phases, which finds extensive application in colloids, phase-change heat transfer (boiling and condensation), microfluidics, and a number of related applications. We present below a summary of TFTL’s research work in the field of colloids and interface science.

Condensation Heat Transfer
Despite the decades of research, surface modification strategy promotes dropwise condensation that are of significant interest in heat exchangers, power plants and water harvesting applications. However, stable dropwise condensation is hardly realized in practical applications due to manufacturing cost, parasitic thermal resistance and long-term robustness of coatings. We at TFTL aim to enhance condensation heat transfer to about 5 times that can demonstrate continuous dropwise condensation for over 1000 hours. The proposed methodology incorporates volatile vapor phase amphiphilic additives which gets adsorbed at the liquid-air interface and hinders the drop coalescence. The amphiphile laden droplet influences repulsion thereby preventing the surface from transition in condensing regimes. Presently, research is devoted towards finding the suitable amphiphiles that can diffuse from vapor phase and adsorb at liquid-air interface of condensing droplets. .

Droplet Evaporation
Evaporation of sessile droplets finds applications in various disciplines of science and technology such as spray cooling, DNA/RNA analysis and inkjet printing. Depending on the contact angle hysteresis, evaporation occurs in distinct modes namely constant contact radius (CCR), constant contact angle (CCA) and mixed. While CCR and CCA modes have been modeled reasonably well, the reasons for the onset of mixed mode and its effect on evaporation time are limited. We performed high-resolution microgoniometry experiments of evaporating sessile droplets on silicon samples contaminated with airborne particles. Dragging of these contaminants along with the receding contact line led us to propose increased surface roughness as one of the reasons for mixed mode of evaporation. We then developed a dynamic roughness ratio based framework for modeling evaporation throughout all the three modes [B5]. Currently, we are investigating evaporation of sessile droplets on lubricant-infused surfaces wherein a meniscus of the surrounding lubricant encapsulates the base of the droplet.

Modeling of Three-phase Contact Line on Microdecorated Surfaces
Modeling of three-phase contact line on microdecorated surfaces Unlike the commonly observed circular shape on smooth and homogenous surfaces, the contact line of a droplet deposited on a microdecorated surface may take various common polygonal shapes such as square, rectangle, hexagon, octagon and dodecagon [B8]. These polygonal contact line shapes are highly stable due to the local energy barriers associated with the anisotropy in de-pining contact angles. We coupled direction dependent dynamic contact angle models from literature [B8] with evaporation models to simulate various polygonal shaped droplets reported in literature [B6]. We are currently studying the effect of anisotropy on apparent contact angles on microdecorated surfaces infused with a lubricant.

Modeling Fluid-Fluid Interface
In TFTL, we have developed a novel spline-based interface modeling and optimization (SIMO) tool to simulate fluidic interfaces. Here, we employ a vector parameterized cubic spline-based representation for droplet profile along with a novel thermodynamic free energy minimization based algorithm to iteratively evolve quasi-steady/equilibrium shapes of pendant and sessile droplets [B7]. SIMO bypasses the cumbersome numerical treatment of the governing Young-Laplace equation and offers an easy and relatively fast method to simulate droplet shapes for a wide range of geometric and surface parameters. We are currently extending SIMO to model interface shapes of sessile droplets on lubricant-infused surfaces.

Modeling Interface Shape on SLIP Surfaces
Recently slippery lubricant-infused porous (SLIP) surfaces have attracted significant attention of the interface science community as they offer excellent liquid repellency. Moreover, such a surface would also facilitate sustained dropwise condensation, which is more efficient in dissipating heat than the commonly observed filmwise condensation. With this motivation, TFTL is currently focusing on modeling interface shapes of sessile droplets on SLIP surfaces [B3]. Unlike, droplets on flat solid surfaces wherein the contact line is easily visible in goniometry experiments, the solid contact line on SLIP surfaces is hidden behind an annular wetting meniscus of the lubricant. This causes the droplet to deviate significantly from the spherical cap shape even at very small volumes. We are currently employing SIMO to model interface shapes on LIS. The effect of meniscus height and droplet evaporation on the apparent wettability are currently underway. Besides the above mentioned major research problems, the ‘Colloids and Interfacial Science’ group is actively engaged with the problems of other research groups in TFTL. Advanced instruments such as microgoniometer (Kyowa Interface Science, Japan) and syringe pump (Cole-Parmer) assist in analyzing the wettability of heater surfaces before and after boiling [A13]. Moreover, boiling with additives such as surfactants, ionic liquids and nanoparticles often necessitates contact angle measurements with such fluids on plain heater surfaces [A6]. In addition, we are also engaged in characterizing surfaces developed by the ‘Energy-Water-Food Nexus’ group for atmospheric water harvesting.

Foams
Foams are the dispersion of gas/vapor bubbles stabilized by surface-active additives dissolved in a liquid medium. Foamability refers to the amount of foam generated and is an important parameter which dictates the choice of additives and their concentrations in various industrial processes and applications such as enhanced oil recovery, food, cosmetics, and heat transfer, among others. Typical foamability characterization strategies are qualitative in nature (foam height or volume) and can only be used for comparisons within the limits of experimental parameters originally tested for. We at TFTL characterize aqueous foaming solutions of surface-active additives to predict foamability solely based on properties of these foaming solutions [B1].

[C] Energy-Water-Food Nexus


Managing the relation between energy consumption, water resource utilization and food availability is key for sustainable development. The increase in global energy demands due to rapid industrialization and urbanization are a serious concern for various developing and underdeveloped nations. The sharp increase in energy demand from 300 EJ to 500 EJ in last four decades has burdened the fossil fuels reserve. Similarly, nearly 2 billion people in rural Africa, Asia, and Latin America have limited access to clean drinking water. Moreover, nearly 45% of fruit and vegetables, 35% of fish and seafood, and 20% of dairy products are wasted in developing and under developed countries due to lack of proper processing and cold storage facility in unelectrified or under electrified regions. In this regard, TFTL group focuses on designing biomass gasification-based energy efficient and environmentally friendly systems. Biomass gasification is the technique to process biomass thermochemically in sub-stoichiometric oxidant environment, leads to generation of combustible gases such as H_2, CO_2 and CH_4. These systems [C1] are envisioned to convert the waste farm residue into useful energy that can be further utilized in various applications such as cold storage [C1], atmospheric water harvesting [C2], space heating, process industries and off-grid server cooling etc.

Biomass Gasification based Thermal Energy Systems
Fossil fuels such as coal, diesel and natural gas are the primary sources of energy which are associated with harmful greenhouse gases and PAH emissions. Moreover, unprecedented industrialization in the past few decades in developing world has immensely burdened conventional energy resources, which has motivated the community to explore viable energy alternatives. Farm residue, a part of biomass is an alternative source of energy with huge potential. It is typically wasted during farm burning, results in severe environmental and health impacts, and soil quality degradation. We, in Thermal and Fluid Transport Laboratory, focus on developing biomass gasification based thermal energy systems to channelize the farm residue to produce useful energy. We recently developed an integrated gasifier-hot water generator (IGHWG) system. This is 22 kW_th , biomass gasification based hot water generation system. IGHWG has an innovative core-annulus arrangement [C3] of gasifier and helical coil heat exchanger which not only promises the complete heat recovery from gasifier surface, burner and flue gases as compared to conventional hot water generation system (separate gasifier and straight tube heat exchanger) but also produce less emissions as compared to direct combustion boilers used earlier.

Applications

Off-the-grid Cold Storage
IGHWG is already in use and commercialized with a 3 TR off-the shelf adsorption refrigeration system with cold storage (named as GreenCHILLTM provided by New Leaf Dynamic Technologies Pvt. Ltd.) to generate cold space. This system is integrated to preserve perishable items such as fruits and vegetables, and ripening of fruits [C1]. Such biomass based cold storages holds unique potential to benefit under-electrified and unelectrified regions globally.

Atmospheric Water Harvesting
Moreover, such system can be conceived to develop environment friendly and off the grid atmospheric water harvesting (AWH) system for the water stressed regions. According to a recent study by our group [C2], such biomass-powered AWH system can produce nearly 800-1200 liters of fresh water per 1000 kg of biomass depending on weather conditions. Such AWH systems would help to alleviate the relevant problems related to water scarcity and environmental degradation facing the today’s world.


Research Facilities

Over the last seven years, we at TFTL have created state-of-the-art research facilities to enable mechanistic insights into complex fluid, interfacial, and thermal transport processes in the areas listed above. The primary focus is to create facilities for scalable micro/nano-fabrication, advanced characterization, state-of-the-art experimentation, and numerical modeling capabilities.

  • High Resolution Microscopic Contact Angle Meter
    Make: Kyowa
    Model: MCA-3

  • High Speed Camera
    Make: Vision Research
    Model: VEO 410L

  • High Speed Camera
    Make: Vision Research
    Model: Phantom v7.3

  • Infrared Camera
    Make: FLIR
    Model: ORION SC7000

  • DC Power Supply
    Make: Agilent
    Models: N5751A (300V, 2.5A), N5749A (100V, 7.5A), N5743A (12.5V, 60A), U8002A (30V, 5A)

  • Data Aquisition Unit with 16 Channel Multiplexer
    Make: Agilent
    Model: 34972A and 34902A

  • Digital Multimeter
    Make: Agilent
    Model: 34460A

  • Workstation
    Make: Lenovo
    Model: P520C

  • Syringe Pump
    Make: Cole Parmer
    Model: 110

  • Hydrophone
    Make: B&K
    Model: 8013

  • Refrigerated and Heating Circulator
    Make: SISKIN
    Model: PROFI CHILL

  • Peristaltic Pump
    Make: Cole Parmer
    Model: 77200-62

  • Ultrasonic Cleaner
    Make: Cole Parmer
    Model: 08895-67

  • Oil Bath
    Make: SEMCO Scientific
    Model: SMOB-20L

  • Fume Hood
    Make: SEMCO Scientific

  • Chemical Cabinet with Exhaust Duct
    Make: SEMCO Scientific

  • Vacuum Pump
    Make: Tarsons
    Model: ROCKER300

  • Vacuum Pump
    Make: Tarsons
    Model: ROCKYVAC811

  • Weighing Scale
    Make: Mettler Toledo
    Model: ME204

  • Degassing Rig
    Make:Custom Built