Effusion Holes Research Articles

Flow and Heat (Mass) Transfer Characteristics in an Impingement/Effusion Cooling System With Crossflow

The present study is conducted to investigate flow and heat/mass transfer characteristics in an impingement/effusion cooling system with crossflow. To simulate the impingement/effusion cooling system, two perforated plates are placed in parallel and staggered arrangements with a gap distance of two times of the hole diameter, and initial crossflow passes between the plates. Both the injection and effusion hole diameters are 10 mm, and the Reynolds number based on the hole diameter and hole-to-hole pitch are fixed to 10,000 and six times of the hole diameter, respectively. To investigate the effect of crossflow, the flow rate of crossflow is changed from 0.5 to 2 times of that of the impinging jet, and the results of impingement/effusion cooling with crossflow are compared with those of the crossflow in the channel and of an array of impingement jets and the effusion cooling system. A naphthalene sublimation method is used to determine the local heat/mass transfer coefficients on the upward facing surface of the effusion plate. The flow patterns are calculated numerically using a commercial package. With the initial crossflow, the flow and heat/mass transfer characteristics are changed significantly from the results without the crossflow. Jet flows ejected from the injection plate are deflected by the crossflow, so that the stagnation points of the impinging jets move downstream. The heat/mass transfer rates on the effusion (target) plate decrease as the velocity of crossflow increases, since the crossflow induces the locally low transfer regions formed at the mid-way between the effusion holes. However, the impingement/effusion cooling with crossflow presents higher heat/mass transfer rates than the array jet impingement cooling with the same initial crossflow.

Local heat/mass transfer and flow characteristics of array impinging jets with effusion holes ejecting spent air

The present study investigates the effects of spent air flows with and without effusion holes on heat/mass transfer on a target plate for array impinging jets. For a conventional type of array impinging jets without effusion holes, the spent air of the injected jets forms a cross-flow within the confined space and affects significantly the downstream jet flow. The injection plate of array impinging jets is modified having effusion holes to prevent the cross-flow of the spent air where the spent air is discharged through the effusion holes after impingement on the target plate. A naphthalene sublimation method is employed to determine local heat/mass transfer coefficients on the target plate using a heat and mass transfer analogy. The flow patterns of the array impinging jets are calculated numerically and compared for the cases without and with the effusion holes. For small gap distances, heat/mass transfer coefficients without effusion holes are very non-uniform due to the strong effects of cross-flow and re-entrainments of spent air. However, uniform distributions and enhancements of heat/mass transfer coefficients are obtained by installing the effusion holes. For large gap distances, the effect of cross-flow is weak and the distributions and levels of heat/mass transfer coefficients are similar for both cases.

Heat treatment in semi-closed ampoule for obtaining stoichiometrically controlled cadmium telluride

The electrical behaviour of CdTe crystals is controlled by background impurity concentrations and stoichiometric deviations. The stoichiometry control is particularly important in vapour growth, in that even extremely reduced deviations in the starting polycrystalline charge give rise to large increases in excess-component partial pressure, thus limiting the crystal growth rate. A novel procedure was developed for preparing polycrystalline CdTe with highly reduced off-stoichiometry. The method is based on a heat treatment of the CdTe charge and on the control of the flux of the Cd and Te 2 vapours through an effusion hole. In situ monitoring of the vapour pressures, and thus of the stoichiometry of the charge was demonstrated. It was shown that the monitoring of the Te 2-partial pressure with time allows to interrupt the process when the desired stoichiometric ratio (Cd/Te≈1 or ≠1) has been reached. Nominally undoped single crystals could be grown from vapour and melt, with a resistivity of 10 8–10 9 Ω×cm, when the Cd/Te atomic ratio was close to unity. A correlation between resistivity and Te 2-partial pressure is reported and discussed.

충돌제트/유출냉각기법에서 분사판의 홀배열이 열전달에 미치는 영향

Two perforated plates are used to investigate local heat/mass transfer characteristics in an impingement/effusion cooling system. A naphthalene sublimation method is conducted to determine the local heat/mass transfer coefficients on the upward facing surface of the effusion plate. Two plates are placed in parallel position with gap distances of 1, 2, 4 and 6 times of effusion hole diameter. The effects of hole arrangements of the plates are studied fur staggered, square, and hexagonal arrays. The experiments are conducted at Reynolds number of 10,000 based on the effusion hole diameter. The results show that the smaller hole size in the staggered array has the higher transfer coefficients on the stagnation region due to the formation of higher momentum flows through the impingement holes. In the square array, heat/mass transfer on the target plate is more uniform as the number of impingement holes increases. High and uniform heat/mass transfer coefficients are obtained for the hexagonal array.

Flow and Heat/Mass Transfer Characteristics of Arrays of Impingement Jets with Effusion Holes

The present study has been conducted to investigate heat/mass transfer characteristics on a target plate fur arrays of circular impingement jets with and without effusion holes. A naphthalene sublimation method is employed to determine local heat/mass transfer coefficients on the target plate. The effusion holes are located at the center of four injection holes in the injection plate where the spent air is discharged through the effusion hole after impingement on the target plate. For the array jet impingement without effusion holes, the array jets are injected into the crossflow formed by upstream spent air because the impinged jets must flow to the open exit. For small gap distances, heat/mass transfer coefficients without effusion holes are very non-uniform due to crossflow effects and re-entrainments of spent air. However, uniform distributions and enhanced values of heat/mass transfer coefficients are obtained by installing the effusion holes. For large gap distances, the crossflow has little influence on heat/mass transfer characteristics on the target palate due to the large cross-sectional open area between the injection and target plates. Therefore, the distributions and levels of heat/mass transfer coefficients are almost the same for both cases.

Local Heat/Mass Transfer Measurement on the Effusion Plate in Impingement/Effusion Cooling Systems

The present study is conducted to investigate the local heat/mass transfer characteristics for flow through perforated plates. A naphthalene sublimation method is employed to determine the local heat/mass transfer coefficients on the effusion plate. Two parallel perforated plates are arranged in two different configurations: staggered and shifted in one direction. The experiments are conducted for hole pitch-to-diameter ratios of 6.0, for gap distance between the perforated plates of 0.33 to 10 hole diameters, and for Reynolds numbers of 5000 to 12,000. The result shows that the high transfer region is formed at stagnation region and at the midline of the adjacent impinging jets due to secondary vortices and flow acceleration to the effusion hole. For flows through the perforated plates, the mass transfer rates on the surface of the effusion plate are about six to ten times higher than for effusion cooling alone (single perforated plate). In general, higher heat/mass transfer is obtained with smaller gap distance between two perforated plates.

Heat Transfer Characteristics on Effusion Plate in Impingement/Effusion Cooling for Combustor

The present study is conducted to investigate the local heat/mass transfer characteristics for flow through perforated plates. A naphthalene sublimation method is employed to determine the local heat/mass transfer coefficients on the effusion plate. Two parallel perforated plates are arranged for the two different ways: staggered and shifted in one direction. The experiments are conducted for hole pitch-to-diameter ratios of 6.0, for gap distance between the perforated plates of 0.33 to 10 hole diameters, and for Reynolds numbers of 5,000 to 12,000. The result shows that the high transfer region is formed at stagnation region and at the mid-line of the adjacent impinging jets due to secondary vortices and flow acceleration to the effusion hole. For flows through the perforated plates, the mass transfer rates on the surface of the effusion plate are about six to ten times higher than for effusion cooling alone (single perforated plate). More uniform and higher heat/mass transfer characteristic is obtained in overall region with small gap between two perforated plates.

The growth of crystals from the vapour

Central to the successful growth of any crystal from the gas phase is control of the vapour transport, something which has proved to be one of the more intractable problems. Much of this difficulty has stemmed from a failure to appreciate and take advantage of the behaviour of gases flowing through pipes and orifices. Additionally, if crystals of high quality are to be produced, then thermal stresses should be kept to the minimum, in particular, contact with the container wall should be avoided. This paper aims to review some of the physical vapour transport methods, paying particular attention to the ways in which the vapour interacts with the container, and with itself and other gaseous species that may be present. Following a brief description of gas flow in ducts and orifices, the paper will consider in turn, closed tube methods, effusion hole techniques, semi-closed methods and restricted flow technologies. In each case, the discussion will aim to establish the salient features of the method and will discuss the extent to which it provides control of vapour transport and growth free of the container walls.

Vapour pressures and standard molar enthalpy of sublimation of crystalline tris(pentane-2,4-dionato)ruthenium(III)

The Knudsen mass loss effusion technique was used to measure the vapour pressure of crystalline tris(pentane-2,4-dionato)ruthenium(III) as a function of temperature between 398.18 and 413.15 K using three different effusion holes. From the temperature dependence of the vapour pressure, the standard molar enthalpy of sublimation at the mean temperature of the experimental temperature range, 405.7 K, was derived as 139.7 ± 2.5 kJ mol −. Using an estimated value of Δ cr g C p o = −50 J K −1 mol −1, the standard molar enthalpy of sublimation at 298.15 K was calculated as 145.1 ± 2.5 kJ mol −1.

Calculation of the beam flux distribution from an effusion hole in the ion source of the mass spectrometer

A calculation method of the molecular beam intensity distribution formed by an effusion hole is presented. This method is applied to the determination of the molecular beam intensity distribution in two versions of an ion source with an electron beam of a mass spectrometer. The theoretical results are compared with results obtained for an atomic silver beam and an “effusion” white light beam (optical model of a molecular beam).

Physical vapor transport growth of mercurous chloride crystals

We have carried out experiments to derive a quantitative understanding of the physical vapor transport (PVT) process and to identify convective effects on the crystal growth process. The experimental growth velocity was several orders of magnitude lower than the theoretically predicted value. The effusion holes were used to disturb the impurity boundary layers. We observed a change of 18% (not an order of magnitude) in growth velocities. The Arrhenius behavior of growth rate with temperature was used to derive the sticking coefficient. Experimental result on growth velocity as the aspect ratio was varied showed that with increasing convection, the growth rate increased up to a certain value and then dropped to a constant value. This indicated that a bifurcation had occured with a resulting change in transport behavior. Growth velocity measurements for the PVT process as a function of orientation of the g-vector were also made. The experimental results clearly showed that the growth velocity varied with g-vector for a particular temperature profile. The effects of convection on crystal quality were studied by varying the thermal conditions (source and crystal temperatures) which affects thermal convection during PVT. The results showed that crystals grown at low Rayleight numbers had better homogeneity. While no microgravity experiments were conducted, computation of mass flux for the horizontal orientation for various gravitational levels showed two distinct regions; above 10 -3 g where the flow was convective and strong circulating cells appeared, and also below 10 -3 g, where the flow was purely diffusive and no circulating cells were predicted. Therefore it is postulated that for the conditions of growth considered, space flight experiments with acceleration less than 10 -3 g could yield crystals grown under diffusive transport.

Lead activity in liquid In-Pb alloy determined from vapour pressure measurements

Analytic pure Aldrich lead (99.9%) and indium (99.98%) were used in the alloy preparation. Stoichiometric amounts of the two components were opportunely melted under vacuum in small quartz tubes up to the temperature of about 600 K, temperature higher than the melting points of both the components and maintained at this temperature for about one day. The liquid alloys so obtained were quenched in air. The lead content of some samples of alloy was checked by chemical analysis and their homogeneity was checked by scanning electron microscopy coupled with X-ray microanalysis. This last technique was also used to check if evident surface depletion of lead surface occurred during the vaporization experiments, particularly when indium rich alloys were studied. The lead activity in seven In-Pb alloys samples was determined by a torsion method following the conventional procedure and also a procedure that permits the direct measurement of the difference between the vapour pressure of the alloy and that of pure lead. The experiments were carried out by employing a torsion graphite cell with the two effusion holes (about 1.2 mm in diameter) drilled on the same side. The torsion assembly is the same as described in a previous work [10]. By filling only one housing (b) with pure lead or indium some vaporization runs were carried out and the results obtained were plotted as shown in Fig. 1. The vapour pressure-tempera ture equations selected in literature [11] for both elements are also plotted in the same figure. The excellent agreement of the second law of vaporization enthalpy, derived from the slopes of the log c~ against 1/T equations, and from the selected logp against 1/T [11], and also the agreement (within the experimental uncertainties of the calibration constants) obtained by employing lead and indium can be taken as a check of the reliability of the used assembly. Using a same housing for vaporizing lead and its alloy, the lead activity was determined from the relation:

Impingement/Effusion Cooling: The Influence of the Number of Impingement Holes and Pressure Loss on the Heat Transfer Coefficient

Measurements of the overall heat transfer coefficient within an impingement/effusion cooled wall are presented. The FLUENT CFD computer code has been applied to the internal aerodynamics to demonstrate the importance of internal recirculation in the impingement gap. This generates a convective heat transfer to the impingement jet. Measurements of this heat transfer plate coefficient are presented that show it to be approximately half of the impingement/effusion heat transfer coefficient. The influence of the relative pressure loss or X/D between the impingement and effusion walls was investigated, for an effusion X/D of 4.67 and a Z of 8 mm, and shown to be only significant at high G where a reduction in h of 20 percent occurred. Increasing the number of holes N in the impingement/effusion array at a constant Z of 8 mm reduced h by 20 percent, mainly due to the higher Z/D for the smaller holes at high N. Reduced numbers of impingement holes relative to the effusion holes, in a ratio of 1 to 4, were shown to have a small influence on h with a maximum reduction in h of 20 percent at high G and a negligible effect at low G.

Electron ionization of the effusion molecular beam emitted by a rectangular channel and the optical model of this effect

Based on previous investigations, the authors have measured the intensity of an N 2 + ion beam, which was generated by crossing an effusion molecular beam of nitrogen with an electron beam. The molecular beam was emitted both from a flat slit and from a rectangular channel. The dependence of the ion current intensity and the electron beam position in relation to the outlet of the effusion hole has been examined. The authors have also demonstrated an optical model of the examined effect.

Standard sublimation enthalpy of condensed GeSe2 by torsion effusion measurements

equal to (0.346 + 0.003) x 10 -s Nm, al and a2 represent the effective orifice areas obtained by appropriate use of the Freeman factors [6] and ll and 12 are the distances from the rotation axis. The three crucibles used were machined from high purity graphite and the diameter and depth of the cylindrical effusion holes were measured microscopically. The constants of the used cells are reported in Table I. Vapour pressures of standard elements (zinc and lead) were determined and compared with the literature [7] in order to test whether the equilibrium thermodynamic conditions, the temperature measurements and the geometrical constants of the used cells were reliable. The employed germanium diselenide was

Experimental thermionic energy converter without enclosure: A molecular beam supplied converter

Experimental thermionic energy converters of an open type, i.e. with no closely located surrounding walls, as proposed here, have many advantages compared to conventional enclosed converters. Interactions with and problems of the enclosure do not interfere with the fundamental plasma and surface processes, which can be studied efficiently in an open converter, for example by mass spectrometry, optical spectroscopy, electron energy analysis and surface analytical methods. An open converter has been built and tested, where the necessary alkali vapour is supplied by a molecular beam type source with 400 laser bored effusion holes. The source acts as collector in front of a directly heated emitter. An ignited mode behaviour can be reached easily. The examples of results presented for Mo and Pt emitters at 1600–1700 K, a Cs plasma and nickel collector give a power density of 1–2 W cm−2 and a minimum barrier index of 1.95–2.1 eV and indicate a behaviour similar to enclosed converters. Thus, results for new surfaces, new additives etc. found in open converters should be of direct relevance for the development of normal enclosed converters.

Vapor pressure of ethyl (5-cyano-3,4-diphenyl-6-oxo-1,6-dihydropyridazin-1-yl)acetate by the effusion method

The effurlon method was used to determine the vapor pressure of ethyl (5cyano-3,4-dlphenyC6-oxo-1,6-dlhydropyridazin-l-yl)acetate (I) at 13 temperatures, in the range 123-141 OC, wlth a mlnhnum of two results at each temperature. Measured vapor pressures were In the range lO-'-6 X lo4 torr and were expreswd by the foilowlng equation: log p = 13.40 - (6891/T), where p Is the vapor pressure in torr and T lo the absolute temperature. The 95% confidence limits for the constants of the equatlon are respectively f1.19 and f486. Experlmental condltlons studied Included effusion hole dlameter and dfuslon thne. Thls compound undergoes no phase change below the meltlng polnt (about 143 C). Ail the results are for the crystalline phase below 143 OC. Some results by the gas saturatlon procedure are also included.

The vapour pressure of tetrachloroisophthalonitrile by the effusion method

The effusion method was used to determine the vapour pressure of tetrachloroisophthalonitrile at 13 temperatures in the range 90-145 degrees C, with a minimum of two results at each temperature. Measured vapour pressures were in the range 1-135 mTorr and are expressed by the following equation for the solid phase of this compound: lg p=12.65-5652.84/T), where p is the vapour pressure in Torr and T is the absolute temperature in K. The 95% confidence limits for the respective constants of the equation are +or-0.3 and +or-122. The average difference between the raw data and the fitted values was 3.4%. Experimental conditions studied included effusion hole diameter and effusion time. The compound undergoes no phase change or decomposition below approximately 167 degrees C, at which temperature there is a solid phase transformation. All the results are for the phase stable below 167 degrees C.

The vapour pressure of 3,3,4,4-tetrahydrothiophene-1,1-dioxide by the effusion method

The vapour pressure was determined at 18 temperatures in the range 30-75 degrees C, with a minimum of two results at each temperature. Measured vapour pressures were in the range 0.3 to 30 mTorr and are expressed by the following equation for the solid phase of this compound: lg p=11.74-(4631/T), where p is the vapour pressure in Torr and T is the absolute temperature in K. The 95% confidence limits for the respective constants of the equation are 0.26 and 85. The average difference between the raw data and the fitted data was 3.8%. Experimental conditions studied included effusion hole diameter and effusion time. The compound undergoes no phase change or decomposition below 143 degrees C, at which temperature there is a loss of HCl.

Zero-Field Emission from Thermionic Hollow Cathodes

An examination of the theory of the thermionic hollow cathode shows that several distinct modes of zero-field emission are possible. The mode that exists in any given case depends on the relationship between a certain parameter d, which characterizes the nature of the electron cloud within the cavity, and the size of the effusion hole and the cavity dimensions. A quantitative expression is given for the conditions under which the hollow cathode can be used for a precise determination of the thermionic constants by the method of Jain and Krishnan. Under other conditions a quasi-hollow beam is obtained and a theoretical prediction of the total zero-field current from the hole was found to be in order of magnitude agreement with experimental observations. An explanation of the similarity of emissions between ‘L’ cathodes of different porosity is also given.