This paper presents a partial review of single-phase experiments & correlations for heat transfer to an array of impinging jets and compares their ability to be predicted by a two-equation numerical ...model for an arbitrary array of jets. It is found that the two-equation numerical model can predict the average Nusselt number with a mean absolute error of 15.8% over the following ranges: Re<105, 0.054mm<dn<8mm, 0.05kg/m2s<mass flux<7000kg/m2s, 0.25<H/dn<30, 1.8<S/dn<330, 1<N<397, with test fluids including air, water, R134a, FC40, and FC77. Using the predictive model, optimization is then carried out and a correlation is found for optimum jet spacing for square heaters with regular rectangular jet arrays.
A model is created to predict the heat transfer performance of oblique and normal jet impingement arrays of arbitrary extent. The simplicity of the model, being based on just two equations, allows ...for ease of implementation and relatively few computations. Comparing the performance of the predictive model with experiment yielded a mean absolute error of 2.9%. Comparison with two frequently referenced correlations in literature show mean absolute errors of 14% and 18% for considerably different test conditions, and comparisons with raw data from these studies have yielded even lower error. The proposed numerical model has a higher accuracy than current CFD implementations and is less computationally involved than full direct numerical simulation.
Two-phase cooling with dielectric fluids of direct-to-silicon devices is hampered by the poor boiling performance resulting from lid removal and poor nucleation behavior of ideally smooth heater ...surfaces. In an effort to maintain thermal capacitance of the working fluid, while maximizing two-phase heat transfer, this study investigates whether liquid pressure gradients near the liquid-heater-interface can be used to increase the active nucleation site density (N''a). Such behavior is preferable to using upstream throttling controls to flash the liquid to saturation as such behavior decreases the available downstream thermal capacitance.A test section and flow boiling loop were built, and could provide an adjustable pressure gradient. Polished float glass, partially coated with ITO, was used for ohmic heating of the fluid while allowing for high-speed imaging of the nucleation behavior. Fluid passing over the glass traveled beneath a converging nozzle, with integrated pressure taps, used to create a linear pressure profile. This nozzle was adjusted to create a range of flow conditions that enabled the effects of Reynolds number and pressure gradient to be separated. Evaluation of the flow loop showed that constant pressure gradients as high as 245 kPa/m could be generated at flow rates approaching 1 L/min.Results showed N''a to be a strong, positive function of the applied heat flux and weak, negative function of pressure gradient. Interestingly, the Reynolds number was shown to be unrelated to N''a for a constant heat flux. No correlation could be found that related the wall superheat to N''a, differing from a significant quantity of existing research based on classical nucleation theory (CNT). These results confirm that CNT does not accurately predict nucleation behavior on ideally smooth surfaces. This research also shows that active nucleation sites are suppressed by 10% at a heat flux of 27 W/cm2 and 34% at 21 W/cm^2 when the magnitude of the negative pressure gradient increases from 163 kPa/m to 240 kPa/m. From the perspective of N''a, minimization of negative pressure gradients should increase the performance of two-phase cooling systems.
Synthesis and detailed characterization of five Tl complexes with three mono-cyanoximes such as phenylcyanoxime, NCC(NOH)C6H5 (further as H(PhCO)), NCC(NOH)C6H4F (later as H(2F-PhCO) and its isomeric ...H(3F-PhCO)), and with two isomeric bis-cyanoximes 1,3-biscyanoxime benzene (later as H2(1,3-BCO)) and 1,4-bis-cyanoxime benzene (later as H2(1,4-BCO)) reported in this work. All compounds represent Werner-type complexes also being extended coordination polymers in which close Thallophilic metal–metal separation was achieved.
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•Preparation of five new Tl(I) coordination polymers with mono- and bis-cyanoximes.•Crystal structures showed non-planar and planar centrosymmetric Tl2O2 dimers.•First time detection of low-frequency Tl–O vibrations in Tl2O2 rhombs.•One compound demonstrated properties of a high-energy compound.
Five new Tl(I) coordination compounds based on aryl monocyanoximes, such as phenylcyanoxime – HPhCO, 1, 2-fluorophenylcyanoxime – H(2F-PhCO, 2, 3-fluorophenylcyanoxime – H(3F-PhCO), 3, of TlL composition, and aryl biscyanoximes, such as 1,3-cyanoxime (benzene) – H2(1,3-BCO, 4 and 1,4-cyanoxime (benzene) – H2(1,4-BCO), 5 of Tl2L stoichiometry were synthesized and characterized using spectroscopic methods, thermal stability studies, and X-ray analysis. All obtained complexes represent coordination polymers of different complexity, ranging in dimensionality from 1D in Tl(2F-PhCO) to 3D in Tl2(1,3-BCO). The most interesting feature of all synthesized complexes is the formation of Tl2O2 rhombs: non planar and non-centrosymmetric in Tl(PhCO), and planar and centrosymmetric in the other three compounds. These rhombi are interconnected, forming zigzag and ladder-type polymers in which very short thallophilic Tl---Tl distances were observed. Thus, in the structure of Tl2(1,3-BCO) the closest distance between metal centers was found to be 3.670 Å. This is the second shortest on-record intermetallic contact in non-organometallic and non-cluster, but Werner type complexes, and is close to that in metallic thallium: 3.456 Å. In all five new coordination polymers the central atom has a stereo-active 6 s2 lone pair that significantly distorts the shape of the coordination polyhedron of Tl(I). The first time, Tl–O vibrations in Tl2O2 rhombs were observed in Raman spectra of the obtained complexes. Thermal analysis studies evidenced stability of all complexes, but Tl(PhCO), to ∼200 °C. The Tl2(1,3-BCO) compound demonstrates properties of the high energy compound, and violently exothermically decomposes at ∼255 °C with the release of a significant amount of kinetic energy. The final product of anaerobic decomposition of all studied Tl-cyanoximates is metallic thallium sponge.
•Bubble formation of Ar on Pt surface with a cavity is studied by MD simulation.•The cavity enhances bubble formation significantly with different argon density.•Argon in the cavity experiences large ...density, repulsive force and energy gradients.•Cavity and smooth surfaces are compared with different surface wettabilities.•Argon atoms are solid-like in the cavity with all the hydrophilic surfaces.
In this paper, molecular dynamics simulations are conducted to analyze the bubble formation in liquid argon on a platinum surface with a cavity. Comparisons are made between the channels with and without the cavity on the lower wall; varying argon densities and surface wettabilities are also examined. The cavity on the surface can significantly enhance the growth of the bubble. The argon atoms in the cavity experience large repulsive forces, repulsive force gradients, density gradients and potential energy along the z axis, which are much larger than those near the plane surface. The argon atoms at the bottom of the cavity seem to be crystallized with all the hydrophilic surfaces, while for the argon atoms near the plane surface, they are solid-like with the strong hydrophilic surface and fluid-like with the weak hydrophilic surface. For the hydrophobic surfaces, there are few atoms in the cavity, and large fluctuations of density are observed near the surface. The plane surface shows almost the same density distributions as the cavity surface, and the surface feature seems to be irrelevant to the bubble formation.
•A novel refrigerant subcooling method is experimentally studied for the first time.•Condensation heat is used to drive an integrated cycle to subcool the refrigerant.•COP and exergy efficiency are ...improved by 18.6% and 27.9% respectively.•Effects of key parameters on the system performance are disclosed.•Economic analysis shows payback period varies from 2.4 to 3.2 years.
Refrigerant subcooling could increase the refrigerating capacity and potentially improve the performance of refrigeration systems. In this paper, a novel subcooling method is experimentally studied for the first time in a hybrid vapor compression refrigeration system. In this system, condensation heat (~40 °C) is used to drive an integrated subcooling cycle to subcool the refrigerant leaving the condenser, which significantly increases the system performance. Changes in system performance are measured as functions of the following variables: the mass flow rates of the dehumidification air, ambient air, dehumidification solution, regeneration solution, and spraying water. Comparisons are made between the proposed system and the traditional water-cooled chiller. The proposed system can achieve a larger degree of subcooling (15–20 °C); what’s more, it shows much higher performances than the traditional water-cooled chiller: COP and exergy efficiency of the chiller are improved by 18.6% and 27.9%, respectively. Performance of the integrated subcooling cycle is also evaluated; it has a low COP, with the maximum value of 0.13, due to the low-grade condensation heat; however, it has a pretty high exergy efficiency, with the maximum value of 0.28, which indicates the effective use of the low-grade heat. In addition, an economic analysis of the integrated subcooling cycle is made with a project life cycle of 15 years; the payback period varies from 2.4 to 3.2 years based on different electricity tariffs, and the savings to investment ratio is between 1.3 and 2.1, which indicates that the project is profitable.
•A novel double temperature chiller with zeotropic refrigerant is proposed.•This chiller can produce chilled water with two different temperatures for THICS.•Effect of refrigerant mass fraction on ...COP and second law efficiency is studied.•Effect of water flow rate and temperature on chiller performance is studied.
A novel chiller with double evaporating temperatures is proposed in this paper, which can be applied in temperature and humidity independent control system (THICS). A zeotropic mixture R32/R236fa is selected as the refrigerant, and chilled water with two different temperatures is produced. The experimental coefficient of performance (COPexp), theoretical coefficient of performance (COPth), and second law efficiency (η) of the chiller are studied. The performance of the chiller is studied by varying the mass fraction of R32 in the R32/R236fa (W(R32)), chilled water temperature, and the flow rates of the heat transfer media (chilled water and cooling water). The results show that the high temperature chilled water (TH,out) can be at 15–18 °C, and the low temperature chilled water (TL,out) can be at 6–8 °C. When TH,out is 17 °C and TL,out is 7 °C, the maximum COPth and COPexp are 4.73 and 3.97, respectively. Second law efficiency, η, increases to 31% as W(R32) increases from 0.3 to 0.6.
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