This grant provides funding for studying and developing a non-intrusive method to enhance local heat transfer between a target surface and a cooling fluid. The approach takes advantage of thermocavitation bubbles, which are formed when continuous wave (CW) laser light is properly focused within highly-absorbing liquid thin films. These bubbles form and collapse within microseconds, vigorously agitating the liquid and enhancing heat transfer. This phenomenon is called Laser-Assisted Surface Cooling Enhancement (LASCE). The PI's preliminary estimates show that heat transfer enhancement is as great as that of phase change (evaporation/boiling), which is the current upper limit to liquid-based cooling systems. LASCE may also match or exceed the performance obtained using micro-modified structures, but without implementing any type of surface modification. Also, CW lasers are often less complex lasing media, cheaper and simpler than pulse lasers, and can control the frequency of bubble formation simply by modulating their output power. The potential impact of this project, therefore, is in novel ways of controlling surface heat. This has implications for systems ranging from laser surgery to combustion engine devices.
This study encompasses collaborative experimental and computational research between University of California-Riverside and University of Wisconsin-Madison aimed at studying and quantifying the: 1) balance between the heat input to induce thermal cavitation and the cooling produced by enhanced convection; 2) sensitivity of the resulting fluid flow to the operating parameters of the laser and; 3) physics of interaction between multiple cavitation events and its effect on enhanced convective cooling. Based on highly accurate numerical simulations and detailed measurement techniques, a thorough examination of the physics underpinning the most desirable heat transfer conditions will be sought, which is essential in optimizing the operation of LASCE. This will be followed by an equally important objective, which aims to explore LACSE's suitability for applications where surface modification is impossible, such as skin (scalp) cooling during laser therapy across a novel transparent cranial implant ("Window to the Brain") developed at UCR. The success of this project will open the door to many transformative applications of optical thermocavitation aimed at low-temperature combustion strategies for mitigating the levels of NOx and particulate matter emissions to improve engine efficiency as well as transdermal drug delivery for diverse biomedical applications.
