Cooling Liquids by Forced Evaporation in BubbleS

Injecting an insoluble gas into a liquid results in subcooling of the liquid, due to forced evaporation into the bubble. While this “bubble cooling” effect causes losses of energy and mass in fluidized-bed reactors, it also can be exploited as an alternative, simple strategy for removing heat from a liquid. Previous studies have assumed the rate of evaporation of the liquid into the bubbles to be independent of the degree of subcooling, but in this study the authors quantify bubble growth directly by high-speed imaging, and disprove this hypothesis. The bubble expansion ratio is a strong function of the liquid’s temperature, as described well by the authors’ model of bubble growth.

Polypropylene materials cooling in liquid nitrogen 

We studied systematically thermal characteristics of polypropylene tubes/vial quenched in liquid nitrogen. Unlike aluminum vials that exhibit prominent film boiling during the initial time of quenching, hardly any film was observed in the case of polypropylene vial and tubes. A one dimensional heat transfer model is developed that also included temperature dependent thermal properties. The resulting approximate solution can predict the temperature along the polypropylene tube wall. The measured temperature of the inner wall of the polypropylene tube and the model prediction are within 8.5 %. The prediction improves as the thickness of the tube wall increases, due to the assumption of perfect thermal contact of the outer wall with liquid nitrogen i.e. the outer wall temperature is equal to the saturation temperature of liquid nitrogen. The model also reveals that the cooling time to a certain temperature is proportional to the square of the wall thickness, which is also experimentally verified.


Plastic vial quenched in a liquid nitrogen bath

impact characteristics of Liquid nitrogen droplets

We studied the impact of single drops as well as a droplet stream of liquid nitrogen on a sapphire smooth target. The transparency and excellent thermal properties of (low temperature) sapphire allowed for total internal reflection imaging as well as isothermal behaviour, even in the contact boiling regime. By varying the drop impact velocity and the target temperature we mapped the boiling behaviour in a phase diagram. Our images also allow to compare the wetted area with the contact line length in the contact and transition boiling regime. These measurements allow us to compare the conductive heat transfer with the evaporation at the contact line. We conclude conduction to be the dominant mechanism, a consequence of the thin thermal boundary layer in the drop. Next, the cooling of the target was studied using the droplet stream. A roughly linear scaling in velocity for the cooling power as well as heat transfer coefficient was found. We have shown that the change of cooling power above a certain target temperature relates to the type of boiling behaviour as observed from our bottom view. This has great implications for understanding of the different heat transport mechanisms in spray cooling.

Heat transfer in Interleaving fins 

We found solutions to the interleaving fins where the fins may develop thermal gradients.  The performance of the fin strongly depend whether or not this effect takes place. We found that the non-dimensional parameter C = ( 4kgL2/ksD (W −D))1/2  characterizes this well, collapsing numerical solutions of the problem when varying C through the different parameters over six orders of magnitude. For C < 1 thermal gradients may be neglected and one should use Eq. 1. In the other cases, we developed and tested an analytical model which showed excellent agreement with both numerical solutions to the problem for a large parameter space as well as with experiments. Next we provided several easy-to-use approximations aimed at system optimization for engineering purposes. Our work offers analytical solution to find the heat flux of this type of heat exchanger and gives new insights, essential in designing and optimizing such systems once the thermal properties are known.





Tissue snap freezer

A snapfreezer to freeze bio-samples enclosed in a vial is developed, which is powered by a low capacity cryocooler. Contact gas and the size of the gas-gap between the vial and the TESU influences the cooling speed. A mathematical model is developed to capture the cooling dynamics of the model, which is then verified with the experimental data. The contribution of heat transfer in the gas-gap due to the advection is small compared to the thermal diffusion through the gas-gap therefore, the gas flow through the gas-gap marginally influences the cooling speed of the vial. We have also shown that the mis-alignment of the vial to the device axis results in the increased cooling speed.

Gas-gap and insulation thermal conductivity measurement apparatus

A single-sided guarded-plate apparatus has been developed to measure the thermal conductivity of gas-gaps or insulation panels of sub-meter size at sub-ambient temperatures ranging from 80 to 300 K. This is a unique apparatus available in the Netherlands.

Cryoablation measurement apparatus

We constructed a test rig to measure the ice ball formation using a cyroablation needle. With our test rig we can vary various parameters to influence the cooling speed. The main objective is to guide the development of a thermodynamic model. A secondary objective is to quantify the cooling power so as to aid the development of more advanced high cooling density needles.