Filmwise condensation of a low surface tension fluid (i.e. refrigerant) on microstructured aluminum surfaces is studied to investigate the effect of the structures on condensation heat transfer at low temperature. The hypothesis is that the structures may cause thinning of the condensate film at micro-scales, thus resulting in an enhancement of condensation heat transfer. However, the structures may also decrease the mobility of the condensate near the surface due to increased friction, thus potentially leading to performance deterioration. The aim of this work is to investigate which of the two counteracting mechanisms dominate during filmwise condensation. Condensation experiments are carried out in a low-temperature vacuum chamber. Compared with the Nusselt model of condensation, the microstructured surfaces, either coated or uncoated, show similar performance, with potentially slight enhancement at low subcooling degree and slight deterioration at high subcooling degree. When the microstructured and silane-coated surface is infused with a non-volatile and very low-surface-tension lubricant oil, the lubricant is displaced by the condensate and there is almost no change in the condensation performance. Our results show that, unlike the case of dropwise condensation of high-surface tension fluids, microstructured and coated surfaces with/without infusing oil is not exciting to enhanced filmwise condensation of low-surface-tension fluids.

Thermal management is essential to compact devices particularly for high heat flux removal applications. As a popular thermal technology, refrigeration cooling is able to provide relatively high heat flux removal capability and uniform device surface temperature. In a refrigeration cycle, the performance of evaporator is extremely important to the overall cooling efficiency. In a well-designed evaporator, effective flow boiling heat transfer can be achieved whereas the critical heat flux (CHF) or dryout condition must be avoided. Otherwise the device surface temperature would rise significantly and cause device burnout due to the poor heat transfer performance of film boiling. In order to evaluate the influence of varying imposed heat fluxes, saturated flow boiling in the evaporator is systematically studied. The complete refrigerant flow boiling hysteresis between the imposed heat flux and the exit wall superheat is characterized. Upon the occurrence of CHF at the evaporator wall exit, the wall heat flux redistributes due to the axial wall heat conduction, which drives the dryout point to propagate upstream in the evaporator. As a result, a significant amount of thermal energy is stored in the evaporator wall. While the heat flux starts decreasing, the dryout point moves downstream and closer to the exit. The stored heat in the wall dissipates slowly and leads to the delay in rewetting or quenching, which is the key to understand and predict the flow boiling hysteresis. In order to reveal the transient heat releasing mechanism, an augmented separated-flow model is developed to predict the moving rewetting point and minimum heat flux at the evaporator exit, and the model predictions are further validated by experimental data from a refrigeration cooling testbed.

Predicting and controlling the flow regime transition of multiphase fluids in microchannels is essential for various energy applications, such as flow boiling, de-emulsification and oil recovery processes. This in turn requires a better understanding of multiphase flow behaviors in microchannels with various channel surface wettability, fluid interfacial tension and flow rates. In this paper, experiments and Lattice Boltzmann method (LBM) simulations are carried out to study complicated multiphase flow at micro or meso scales. With the Shan-Chen multiphase LBM model, the flow pattern transitions of adiabatic two phase flow in a microchannel were investigated. The effects of surface wettability and liquid/gas velocity ratio on the flow regime transition were further studied. A series of two-phase flow experiments were conducted on a PDMS microfluidic device under different gas/oil velocity ratios. Under various surface wettability conditions, our simulation results agree well with the flow visualization experiments equipped with a high speed camera (HSC). Our finding shows that the cross-section meniscus curve width, corresponding to the shadow in the HSC photo, increases with decreasing contact angle, which was confirmed by the simulated liquid/gas distribution. Besides the influence of surface wettability, the role of gas/liquid velocity ratio on two-phase flow regime transition was discussed in detail. The proposed approach paves the way to probe complicated physics of multiphase flows in microporous media.

Jumping-droplet enhanced condensation has recently attracted huge interest due to its remarkable potential of heat transfer performance enhancement, and studies have been done to design superhydrophobic surfaces with various surface morphologies. We fabricated a superhydrophobic micromesh-covered surface using a facile and scalable method. ESEM condensation experiment results show that droplets in pores formed by the mesh wires had faster growth rate in the upward direction than droplets on wires. This is mainly because of the confining role of the wires and higher heat transfer rate due to larger solid-liquid contact area. Also, these droplets always jumped at the size of pores (∼35 μm) when they coalesced with other droplets on wires. Moreover, droplets in pores were distorted by mesh wires, resulting in larger surface area. Theoretical predictions show, for a specific droplet radius, coalescence jumping of distorted droplets on the mesh-covered surface releases more excess surface free energy, and has larger jumping velocity than that of spherical droplets on the plate surface without mesh. This better performance was further validated by constant exposure of those two surfaces to electron beam during which work of adhesion was gradually increased. As expected, droplets on the mesh-covered surface coalesced and jumped while coalescing droplets on the plate surface could not as the exposure time increased. Our results offer new insights for designing hierarchical structured superhydrophobic surfaces to further enhance the performance of condensation heat transfer processes.

Rarefied gas flow plays an important role in the design and performance analysis of micro-electro-mechanical systems (MEMS) under high-vacuum conditions. The rarefaction can be evaluated by the Knudsen number (K n ), which is the ratio of the molecular mean free path length and the characteristic length. In micro systems, the rarefied gas flow usually stays in the slip- and transition-flow regions (10 −3 < K n < 10), and may even go into the free molecular flow region (K n > 10). As a result, conventional design tools based on continuum Navier-Stokes equation solvers are not applicable to analyzing rarefaction phenomena in MEMS under vacuum conditions. In this paper, we investigate the rarefied gas flow by using the lattice Boltzmann method (LBM), which is suitable for mesoscopic fluid simulation. The gas pressure determines the mean free path length and K n , which further influences the relaxation time in the collision procedure of LBM. Here, we focus on the problem of squeezed film damping caused by an oscillating rigid object in a cavity. We propose an improved LBM with an immersed boundary approach, where an adjustable force term is used to quantify the interaction between the moving object and adjacent fluid, and further determines the slip velocity. With the proposed approach, the rarefied gas flow in MEMS with squeezed film damping is characterized. Different factors that affect the damping coefficient, such as pressure of gas and frequency of oscillation, are investigated in our simulation studies.

In practical control systems, the plant states are not always measurable, so state estimation becomes essential before the state feedback control is applied. In this paper, we consider output feedback model predictive control (MPC) for linear parameter varying (LPV) systems with input constraints. We proposed two approaches to obtain the observer gain, that is to compute the gain in the dynamic optimization at each time instant (on-line), and to compute the gain in advance (off-line), respectively. By applying both approaches, the state estimation error goes to zero asymptotically, meanwhile, the state feedback gain is optimized. In fact, the on-line approach can help enlarge the feasibility region and improve the control performance. It has been shown that feasibility of both approaches can be maintained for the closed-loop control systems even in the presence of state estimation error. Finally, the proposed output-feedback MPC strategies are applied to an angular positioning control system and the control of a transcritical CO 2 vapor compression refrigeration system.

A combined thermal power and ejector refrigeration cooling cycle is proposed in this paper to harness low-grade solar energy. It utilizes abundant and low-cost hydrocarbon as the working fluid. Hydrocarbon has been identified as a promising alternative to existing high global-warming-potential refrigerants (i.e., HFC refrigerant R134a) in next-generation refrigeration systems. Several typical alternative refrigerants are evaluated by considering their fundamental thermophysical properties: absolute pressure level, volumetric cooling capacity, surface tension, saturated liquid/vapor density ratio and kinematic viscosity. Comparing with R1234yf, R1234ze and R744 (CO 2 ), hydrocarbon refrigerants, such as R290 (propane) and R601 (pentane), do have inherent advantages for either cooling or power generation purposes in hot climates: lower flow resistance and better heat transfer at higher temperature. Fundamental phase stability and transition issues have been considered in designing pentane vapor ejectors for combined power and cooling cycles operating at high ambient temperature. Thermodynamic analysis has indicated that the proposed solar thermal system can provide an effective way to sustainable energy production in hot and dry climates.

Compressor is the main component in the Vapor Compression Cooling cycle (VCC). Comparing with scroll and screw compressors, linear compressors are used in miniature-scale VCC cycle for small and portable applications such as electronics cooling systems. Linear micro-compressors exhibit high performance because they have fewer moving components and less frictional losses than other types. In this paper, a first-principle dynamic model has been developed to characterize the transient pressure, temperature, and fluid flow inside a linear micro-compressor. A theoretical analysis and a parametric study have been performed in this research to reveal the influence of the piston motion profile on the pressure and temperature changes inside the compression chamber. Moreover, the effect of changing piston velocity on the flow rate, pressure and temperature trends has been studied in this research.

Experimental investigations are conducted to determine the two-phase frictional pressure drop of refrigerant R134a and the thermal entrance length for flow in a helically coiled mini-tube of 2mm inner diameter. The objective of this work is to study the effect of the tube curvature on the frictional pressure drop in two-phase flow as well as the thermal entrance length in laminar single phase flow. The two-phase frictional pressure drop is investigated through flow boiling of the refrigerant under uniform wall temperature boundary conditions. A generalized correlation is proposed to predict the single phase and two-phase frictional pressure drop through the coiled tube. A large set of experimental data is collected to evaluate the prediction performance of the proposed two-phase correlation. The results of the prediction are fairly good when compared with the measured pressure drop from experiments. A Computational Fluid Dynamics (CFD) analysis is also performed to compare with the thermal entrance length study from experiments. The results from the experimental analyses and CFD are seen to be in good agreement. It is found that the thermal entrance length for the coiled tube is 27–103.3% larger than a straight tube of equal length and cross-section.

Vapor compression refrigeration (VCR) cooling has been identified as a promising solution to ensure the low-temperature sustainable operation of photonics, avionics and electronics in extreme hot weather. With the inherent benefits of saturated flow boiling in a direct VCR cooling cycle, uniform low surface temperature and low solid/liquid thermal resistances can be achieved. However, flow boiling heat transfer performance is limited by the relatively low critical heat flux (CHF) condition because the evaporator inlet flow is already a liquid/vapor mixture. Moreover, for the aforementioned applications, the dissipated heat loads are usually subject to large and transient changes, which could easily cause the evaporating flow to exceed the CHF point. Therefore, it is important to characterize boiling heat transfer in transient VCR evaporators under both pre-CHF and post-CHF conditions. Comprehensive experimental data are reported in this paper to describe the complete forced convection boiling hysteresis at the evaporator exit. Several well-known boiling heat transfer correlations and flow pattern criteria are used to help understand the physics of the hysteresis. An empirical model is developed to reveal the unstable nature of transition flow boiling dynamics. A probability distribution function model is further proposed to predict the droplet size in mist flow and vapor core of annular flow. This study provides more design and operating guidelines for the application of saturated flow boiling systems in renewable power generation and electronics/photonics/avionics cooling industries.

For next-generation sustainable electronic systems, such as high-concentration photovoltaics arrays and high-density super-computers, two-phase cooling technologies are being explored to significantly reduce heat resistance from electronics’ surface to the ambient. Lower electronics operating temperatures lead to higher energy conversion or computation efficiency; therefore, thermal management, especially dynamic thermal management, is able to bring great potential to energy-efficient electronic system operation. These large-scale electronics cooling systems normally include multiple, distributed, and transient heat sources. Multi-evaporator vapor compression refrigeration cycle provides such a promising cooling solution. Due to the complexity of multiple evaporator structure, its transient analysis and active control become very challenging. This paper applies our previous distributed heat exchanger modeling techniques to study the dynamics of multi-evaporator refrigeration cycles. A comprehensive first-principle multi-evaporator vapor compression cycle model is formulated for its transient analysis. Some preliminary expansion valve control results are presented to show the excellent active electronics cooling capability. This general tool is expected to bring instructive guidelines for the optimal design and operation of energy-efficient transient electronics cooling systems with multiple heat loads and hot spots.

For high-power electronic systems, such as high-concentration photovoltaics arrays, laser diode arrays, and high-density data centers, two-phase cooling technologies are being explored to significantly reduce heat convection resistance from electronics’ wall to the ambient. Lower electronics surface or junction temperatures lead to higher energy conversion or computation efficiency; therefore, thermal management is a critical issue for energy efficient electronic system operation. In large-scale electronics cooling systems, there usually exist many distributed and transient heat sources. The non-uniform heat loads could cause severe flow mal-distribution problems and local device burn-out (i,e, a difficult thermal management challenge). Vapor compression refrigeration cycles have been identified as promising solutions to ultra high-power electronics cooling. A well-designed active refrigeration cooling system is expected to achieve higher transient cooling capability and energy efficiency. This paper presents a comprehensive first-principle dynamic refrigeration cycle model to understand its fundamental mass, energy, and momentum transport mechanisms in transient operation. Experimental validation results show the proposed distributed vapor compression cycle model has excellent steady-state and transient prediction performance. The proposed distributed dynamic model is able to provide valuable design and operation guidelines for energy-efficient electronics cooling systems under transient and non-uniform heating scenarios.

In this paper dynamic identification of the evaporator dynamics in a vapor compression cycle (VCC) subjected to imposed heat flux is studied. The imposed heat flux boundary condition at the evaporator represents a specific application of the VCC for electronics cooling. However, different models and control algorithms than traditional VCCs are required. First principle models are highly nonlinear and, hence, not practical for system control. A dynamic model identification of the refrigerant temperature at the exit of the evaporator, refrigerant pressure, and temperature of the heating element is performed by varying the expansion valve opening. It is shown that single-input single-output (SISO) identification is not sufficient to capture the dynamics of the evaporator, due to the coupling of the dynamics in the entire system. Including the effect of incoming mass flow rate into the evaporator to the model significantly improves the identification and prediction of the evaporator dynamics. Finally, a SISO controller based on the identified model, is designed and tested experimentally. The control objective is to maintain the temperature of the heating element below a set point, subjected to changes in heat flux.

In this paper dynamic identification of the evaporator dynamics in a vapor compression cycle (VCC) subjected to imposed heat flux is studied. The imposed heat flux boundary condition at the evaporator represents a specific application of the VCC for electronics cooling. However, different models and control algorithms than traditional VCCs are required. First principle models are highly nonlinear and, hence, not practical for system control. A dynamic model identification of the refrigerant temperature at the exit of the evaporator, refrigerant pressure, and temperature of the heating element is performed by varying the expansion valve opening. It is shown that single-input single-output (SISO) identification is not sufficient to capture the dynamics of the evaporator, due to the coupling of the dynamics in the entire system. Including the effect of incoming mass flow rate into the evaporator to the model significantly improves the identification and prediction of the evaporator dynamics. Finally, a SISO controller based on the identified model, is designed and tested experimentally. The control objective is to maintain the temperature of the heating element below a set point, subjected to changes in heat flux.