| With the continuing increase in the functionally and compactness of microelectronics, the demand for high heat flux dissipation from the high performance microprocessors increases. So the development and application of the cooling technique becomes the primary task to maintain the excellent performance and stability of the microelectronic device. Recently, microscale two-phase boiling heat transfer provides an alternative and potential way for thermal management of high-density commercial and defense electronic devices. To provide a guide to design and optimize the two-phase boiling heat transfer in microchannels, in this thesis, the acetone is used as the working fluid, and the experimental study is performed to study the mechanism of boiling flow instability in microchannels. At the same time, the seed bubble-triggered evaporation heat transfer and the seed bubble-depressed boiling instability models are developed and numerically studied. The main contents are as follows:Effect of running parameters on flow boiling instability in microchannels. The parallel triangle silicon microchannel heat sink is designed as the experiemntal section, and three different lengths (L1=4.5mm, L2=6.25mm, L3=8.0mm) for the main heater on the back of the heat sink are used. The demand curves of pressure drops (?P) and temperatures at the central point of the main heater (TWC) versus heat fluxes are examined for different mass fluxes and heater lengths. It is found that, both the single-phase liquid region and two-phase flow region are included in the plots. And in the single-phase liquid flow, the pressure drops slightly decrease and the main heater temperatures linearly increase with the heat flux increasing. And in the two-phase flow region, the pressure drops rapidly increase and the main heater temperatures exponentially increase with the heat flux increasing. The effects of heat fluxes(q), heat lengths(L), and mass fluxes (G) on pressure drops, main heater temperatures, the standard temperature deviations (Stdev), and the heat transfer coefficients (hWC) are experimently studied. The results show that, for the higher heat flux/lower mass flux/longer main heater length, the boiling instability is earlily triggered, the standard temperature deviations are decreased, the pressure drops are increased, also the cycle lengths and oscillation amplitudes of main heater temperatures and heat transfer coefficients are increased.Experimental study of seed bubble-depressed flow boiling instability in microchannels. Seed bubbles can be considered as a type of intelligent and controllable'cavity'bubbles to control the flow boiling instability, which were producted on a set of microheaters upstream of microchannels driven by the pulse voltage signal. The effect of seed bubble frequencies (f) on boiling flow instability is experimentally studied. It is found that, the oscillation amplitudes of the pressure drops and main heater temperatures become smaller with the seed bubble frequency increasing. And the heat transfer enhancement attains the maximum degree for f=1000Hz, inferring that a saturation frequency of1000Hz for the corresponding mass flux. It is noted that for the middle boiling number of1.5×1e-03-3.21e-03there is only one saturation frequency. The demand curves of pressure drops (?P) and temperatures at the central point of the main heater (TWC) versus heat fluxes are investigated under different seed bubble triggering frequencies. It is found that, the seed bubble has no effect for the flow and heat transfer in the single-phase liquid flow. In the two-phase flow region, the slope of the pressure drops and the main heater temperatures decreases with the heat flux increasing, inferring to the progressive control of the boiling flow instabilities. The effects of seed bubble frequencies(f), on pressure drops, main heater temperatures, the standard temperature deviations, and the heat transfer coefficients are experimently studied. The results show that, with the seed bubble frequency increasing, the cycle lengths and oscillation amplitudes of pressure drops and main heater temperatures and heat transfer coefficient are decreased, the standard temperature deviations is increased. Naturally the heat transfer enhancement is induced. With the assistant of high frequency seed bubbles, the typical flow patterns in microchannesl can be converted from the alternating single-phase liquid flow and annular/mist flow to the quasi-stable slug flow.Numerical study of seed bubble-enhanced evaporation heat transfer in microchannels. Selfboiling in microchannels was ignored in this chapter, The seed bubble-triggered evaporation heat transfer model is in microchannels developed. A simple but accurate phase change model for vapor-liquid two phase flow proposed by our research team is used to describe the evaporation mass flux on the vapor-liquid interface. To reduce time consumption and avoid data transmission error at the allocation interface, the fixed grid allocation technique is proposed to successfully perform the parallel computation via a set of computer core solvers. The wall superehats (?Tw,x) and bulk superheats(?Tb,x) versus time under different triggering frequencies, and the relationship between the flow pattens and heat transfer are explored. It is found that the seed bubble frequency is a key parameter to influence the heat transfer performance. Low-frequency seed bubbles cause apparent spatial-time oscillations of wall and bulk superheats. High-frequency seed bubbles result in quasi-stable elongated bubbe flow, corresponding to quasi-uniform and stable wall and fluid superheats. And the evaporation of thin liquid film between the elongated bubble and the wall is responsible for the heat transfer enhancement. In addition, the experimental evidence that the saturation seed bubble beyond which no further performance improvement can be made is also verified by the numerical model, and the seed bubble-triggering evaporation heat transfer mechanism in ms-level, which is difficult to capture by the limitation of measurement technique, is successfully revealed by the present numerical study.Numerical study of seed bubble-depressed flow boiling instability in microchannels. Selfboiling in microchannels is considered in this chapter. Based on the experimental conclusions by our research team and a series of assumptions, the seed bubble-depressed flow boiling instability vapor-liquid two phase model for middle boiling number is developed. The relevant parameters for the model consist of the nucleation criterion(?Tw), nucleation-frequency (fn), nucleation density (Nn), and seed bubble triggering frequency(/). The wall superheats (?TW,x), bulk superheats(?Tb,x), pressure drops(AP) and enhanced heat transfer coefficient((Nux-Nufull)/Nufull) versus time for different mass fluxes (G) are examined. It is found that the wall superheats, bulk superheats, pressure drops and enhanced heat transfer coefficient varies synchronously with apparent oscillation amplitudes and cycle lengths. With the mass flux increasing, the nucleation frequency is increased and the first nucleate point is more close to the microtube entrance. And the effect of seed bubble frequencies on the flow boiling instability and heat transfer enhancement under different mass fluxes is also studied. It is found that, the intense oscillation of the wall superheats, bulk superheats, pressure drops and enhanced heat transfer coefficient is successfully depressed, corresponding to a nice quasi-stable spatial-time distribution. However, the saturation seed bubble frequency is not changed with the mass flux increasing. And for the same saturation frequency, the residual wall and bulk superheats increase with mass flux increasing, when the latter reaches the required minimum temperature difference to maintain the evaporation heat transfer, the ingored change will be induced. |
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