dr. S. Vanapalli (Srinivas)

RESEARCH INTERESTS

Dynamical systems, Energy systems modeling, Cryogenics & Cryomicroscopy, Multiphase fluid and heat transfer, Experimental fluid mechanics, MEMS.

RESEARCH HIGHLIGHTS

Multi-bilayer thermal storage
A multi-bilayer thermal storage system, consisting of alternate layers of phase change material and thermal insulation decreases the overall thermal diffusivity. Multi-bilayers are experimentally proven to have more than 200 % increase in hold time for a three bilayer system compared to a single bilayer system, considering the same weight for both systems. 

Vanapalli, S. & van der Leij, T., WO 2016/162451, International patent, October 13, 2016.

Compact additive manufactured thermal device
A compact flat-panel gas-gap heat switch is developed with additive manufacturing. The novelty in this approach is that several components can be produced simultaneously (e.g. fluidic interconnects and heat sink), and allows three dimensional flexibility in the design of the outer structure to tailor the device for any intended application (either flat, cylindrical or any other shape). It has clear potential for quick adoption by stringent markets, because of its low footprint and mass, and without moving parts or welds.

Vanapalli, S.  et al. Cryogenics 78, 2016; 
Krielaart, M.A.R., Vermeer, C.H., Vanapalli, S., Review of Scientific Instruments 86, 2015.

Cryogenic optical microscopy
A tool to visualize the deposition and sublimation of impurities (water vapour) in a narrow slit of 1 micron depth at cryogenic temperature (around 150 K) is developed. A tailor made glass cryostat and a glass micro-machined Joule Thomson cold stage is used to study the heat and mass transfer process in the microchannels. For the first time, such visualization experiments are performed in the context of cryogenic coolers. The optical images, temperature recording and the mass flow data gives insight into the physical process that influence the (de-) sublimation process.

Phd thesis: Cao, H.S., University of twente, 2013;
Cao, H.S., Vanapalli, S.  et al., Applied Physics letters 103, (2013)

An apparatus to measure thermal conductivity of insulation panels at sub-ambient temperature
A single-sided guarded-plate apparatus has been developed to measure the thermal conductivity of insulation panels of sub-meter size at sub-ambient temperatures ranging from 250 to 300 K. This apparatus allows thermal conductivity measurements to be performed at large temperature differences simulating the actual operating conditions in an application and can accommodate panels of various thickness. The cold plate in the apparatus is cooled with a flow thermostat.

Vanapalli, S. et al. International Journal of Refrigeration 74, 2017.

Assessment model for nanofluids
Nanofluid is a suspension of nanoparticles in a carrier fluid. A simple thermal performance assessment model based on first principles is developed that can be used by industry to rapidly evaluate the thermal performance of a nanofluid in comparison to its base or carrier liquid in convective systems. The model takes into consideration a number of key properties of the base liquid and the nanoparticles dispersed including changes in viscosity, thermal conductivity, specific capacity and concentration of the nanoparticles, density as well as the flow rate, geometry and flow conditions. Hence, the model can be used by industry to optimize both the formulation of the nanofluid and geometry of the heat transfer unit deployed to harness the benefits.

Vanapalli, S. & ter Brake, H.J.M., International journal of heat and mass transfer 64, 2013.
Vanapalli, S. et al. International Journal of Refrigeration 74, 2017.

Micro cryogenic Joule Thomson cryocoolers
Using the state-of-the-art glass microsystems technology (isotropic chemical etching and bonding wafers), a two stage Joule Thomson cold stage (PhD work of H.S.Cao) with a temperature of 30 K is developed, and have demonstrated the application of this cooler by attaining superconductivity of a thin film device.

Cao, H.S., et al., Int. J. Refrigeration, 2016.
Cao, H.S., et al., Journal of Micromechanics and Microengineering, 23, 2013.
Cao, H.S., et al., Cryogenics, 52, 2012.

Thermoacoustic heat pump
A system for a dwelling, which is based on thermoacoustics and can be used to switch the role between a heat pump and a cooler as the need arises, has been developed. The system uses an environment friendly working medium and was shown to have a reasonably good performance. This work is performed at the Energy research Center of the Netherlands.

Vanapalli, S., Tijani, M.E.H., Spoelstra, S., ASME 2010 Summer meeting.

High frequency pulse tube cryocooler
A pulse tube cryocooler operating at 120 Hz with 3.5 MPa average pressure achieved a no-load temperature of about 49.9 K and a cooldown time to 80 K of 5.5 min. The net refrigeration power at 80 K was 3.35 W with an efficiency of 19.7% of Carnot when referred to input pressure-volume (PV or acoustic) power. Such low temperatures have not been previously achieved for operating frequencies above 100 Hz. The high frequency operation leads to reduced cryocooler volume for a given refrigeration power, which is important to many applications.

Vanapalli, S., Lewis, M., Gan, Z., Radebaugh, R.,Applied Physics Letters 90, 2007.

Publications

• 201700091 - Thermodynamics - AT
• 201800158 - MOD 02 TN Thermodynamics
• 201800334 - A first practical course in Electronics

Organizations

• Faculty of Science and Technology (TNW), Energy, Materials and Systems (EMS)