The influence of microstructured surfaces and liquid pulsations on increasing heat transfer in a minichannel

. The paper compares two methods for improving the intensification of heat transfer during boiling in a rectangular minichannel. Studies were carried out of the influence of additive structures on the crisis of heat transfer in a minichannel with a two-phase flow, as well as studies of the influence of high-frequency pulsations of liquid flow on the intensification of heat transfer processes during flow boiling.


Introduction
Solving the global problem of intensive heat transfer flows is more urgent than ever.The development of current and next-generation high-performance electronic devices has led to using electronics with smaller transistor sizes.As a result, the density of transistors per unit area increases, and eventually, the heat flux density increases as well.Therefore, it is necessary to develop new liquid heat transfer technologies.The use of microstructured surfaces and pulsating flow in single/two-phase cooling systems may be a potential solution to problems caused by high heat flux densities.
Recently, one of the most trending topics is heat transfer during boiling with flow oscillations; researchers have mainly considered unidirectional flows with a change in flow velocity.However, the experimental conclusions vary and even contradict each other due to different approaches to obtaining pulsations and experimental conditions [1].Kaern et al. [2,3] experimentally study the boiling of a pulsating flow of R134a with a 1-9 s period.They find that the effect of flow pulsations on the heat transfer coefficient strongly depends on their period: with a short period (1-2 s), it increases, and with an extended period (8-9 s), it decreases.Wang et al. [4] demonstrate a similar result, using electromagnetic valves to create square waves with a 2-20 s period.They observe an increase in the heat transfer coefficient up to 27%; with an increase in the pulsation period, it decreases.Some researchers conclude that the frequency of pulsations does not significantly affect heat transfer processes; others consider the flow pulsations as a deteriorating factor in heat transfer.Zhao et al. [5] examine the heat transfer coefficient during boiling in a pulsating flow with a 1.04-10.6s period.They discover that when boiling a two-phase flow, the local heat transfer coefficient is practically unaffected by a flow pulsation at the inlet.The critical heat flux worsens in a pulsating flow and decreases with increased amplitude and period of the oscillations.Okawa et al. [6,7] present a similar result as they investigate the effect of an oscillatory flow on the critical heat flux with a 2-20 s period.
This research conducts experimental analyses on the intensification of heat transfer processes in a system with boiling in a microchannel on microstructured surfaces without pulsations and on a smooth heater using controlled fluid flow pulsations.It compares the effect of such pulsations on the critical heat flux during liquid boiling in a minichannel with various local heating sources, which simulates a microelectronic chip's performance.The authors demonstrate the cooling efficiency of a system with controlled liquid flow pulsation compared to liquid boiling in a constant flow minichannel.

Experimental setup
The experimental setup includes a working section, a liquid circuit, measurement and control systems, Fig. 1.The liquid circuit contains a programmable liquid pump (Ismatec Micropump with a Z-183 MI 0008 head), with an operating liquid flow range from 0 to 100 ml/min.The pump can generate liquid pulsations from 0 to 10 Hz frequency.Milli-Q ultrapure distilled water was used as the working fluid.
The setup contains temperature measurement equipment: a system of K-type thermocouples for measuring the surface temperature of the working area and an IR camera "Titanium HD 570M" for measuring the temperature field of the surface.In addition, the stand is equipped with a control and measurement system "National Instruments", that allows the control of a liquid pump (set the frequency and amplitude of pulsations) and record data from thermocouples and pressure sensors (Wika P-30) on a PC.The working area is a rectangular minichannel (height -0.9 mm, width -30 mm, length -59 mm).The channel is located horizontally.The diagram of the working area is shown in Fig. 2. The lower part of the channel is a flat stainless steel plate with a heater (smooth or with microstructures) installed flush into it, which is used as a source of local heating.The surface area of the heater is 10x10 mm 2 .The heater surface can be smooth or with microstructures (Fig. 3).The thermal conductivity of the additive surface is measured by laser flash method by LFA-427 (NETZSCH -Germany) in temperature range 298-673 К (Fig. 3).A power supply is 3.We carried out a simultaneous comparison of visual, thermal, and temperature data during the experiment.In the work, we monitored the behavior of the heater temperature, the temperature at the entrance to the working section, the temperature distribution on the surface of the working section, as well as the ongoing processes in the channel.During heat transfer crisis experiments, we manually set the current and voltage on the heater using the power supply fine-tuning mode.We controlled the establishment of a stationary temperature on the surface of the working area.A highspeed camera is used to film the working area.At the time of the crisis, we observed a sharp increase in the surface temperature of the heater, took a control series of pictures, and then turned off the heater for safety reasons.Thus, after one experiment, we obtained a set of photographs indicating the current temperature and heat flow, which built the basis for the following data analysis.

Results and discussions
In the course of an experimental study on the effect of fluid flow pulsations on the enhancement of heat and mass transfer during boiling flow in a rectangular minichannel, the authors found that for frequencies from 0.2 to 1 Hz, the increase in heat transfer could reach 25-30% compared to the mode without pulsations [8] The research showed that at pulsation frequencies of the fluid flow less than 0.14 Hz, the gain was either absent or reduced compared to the mode without pulsations.The experiments were carried out at the same average fluid flow rates in the system.The development of a crisis in a pulsating flow regime has been shown by visualizing the surface of a local heater in a minichannel using an IR camera.Periodic rewetting and the appearance of dry patches corresponded to the frequencies of changes in the superficial velocities of the liquid.The pulsating flow regime at high frequencies delayed the onset of a crisis in the system.This phenomenon led to a periodic increase in surface temperature, though it did not turn into a full-fledged crisis.Flow pulsations due to a short-term increase in the superficial velocity of the liquid resulted in rewetting dry patches.For a pulsating flow regime, the classical transition of the system to a boiling crisis was observed -as the heat flow increased, the area of the dry patch grew until the entire surface dried out, and the system went into a crisis with a sharp increase in surface temperature [8].
The study [8] demonstrated that using a pulsating flow mode increased heat transfer by 20-30% in wide minichannels with a local heat source without significant complications and technical improvement of the cooling system only due to pulsation.At the same time, temperature changed over time (up to 20 ⁰C) could not provide stable operation conditions for all technical applications.
During experimental study on the influence of additive structures on the heat transfer crisis in a rectangular minichannel [9], the authors compared two heaters -a smooth one and one with microstructures.On a smooth heater, they observed the formation of tiny bubbles, and then, with a further increase in the heat flow, merging with the formation of a large one and, consequentially, a heat transfer crisis, Fig. 5.
On a heater with microstructures at low heat fluxes, bubbles merged and migrated to the side walls due to capillary forces, Fig. 6.
The research discovered that the efficiency of heat transfer was higher when using a heater with microstructures (because of a large number of evaporation areas due to the manufacturing features of the heating surface).Furthermore, experiments have shown that the heat removal efficiency was higher in the case of a single-phase flow for an additive heater, Fig. 7 [9].
For pulsation frequencies of fluid flow 0.2 -1 Hz increase in heat transfer can reach 25-30% compared to pulsation-free mode.At frequencies less than 0.14 Hz gain or no gain, or decreases compared to pulsation-free mode.Experiments were carried out at the same average fluid flow in the system.

Conclusions
In this research has shown that the use of surfaces with microstructures together with pulsations of fluid flow will enhance heat transfer in the system.
3 kW DC.The working fluid enters through a fluid nozzle located at an angle of 11°.The inlet temperature of the working fluid does not exceed 25⁰ C. The upper part of the channel is an optical glass installed to monitor the processes in the channel.

Fig. 6 .
Fig. 6.Formation and rewetting of a dry patch on the surface of a heater with microstructures, Vsl = 0.006 m/s, 60 W, the flow approaches from the right.