Prediction of CHF in concentric-tube open thermosiphon using artificial neural network and genetic algorithm

In this paper, an artificial neural network (ANN) for predicting critical heat flux (CHF) of concentric-tube open thermosiphon has been trained successfully based on the experimental data from the literature. The dimensionless input parameters of the ANN are density ratio, ρ _l/ρ _v; the ratio of the heated tube length to the inner diameter of the outer tube, L/D _i; the ratio of frictional area, d _i/(D _i + d _o); and the ratio of equivalent heated diameter to characteristic bubble size, D _he/[σ/g(ρ _l−ρ _v)]^0.5, the output is Kutateladze number, Ku. The predicted values of ANN are found to be in reasonable agreement with the actual values from the experiments with a mean relative error (MRE) of 8.46%. New correlations for predicting CHF were also proposed by using genetic algorithm (GA) and succeeded to correlate the existing CHF data with better accuracy than the existing empirical correlations.     

Critical heat flux during natural convective boiling on uniformly heated inner tubes in vertical annular tubes submerged in saturated liquid

An experimental study has been made of critical heat flux (CHF) of natural convective boiling on uniformly heated inner tubes in vertical annular tubes. The experiment was performed at a pressure ofP=0.1 to 3.1 MPa for the clearance of 0.4 to 4.0 mm, the heated tube diameter of 5 to 10.6 mm, the annular tube length ofL=58 to 840 mm and three kinds of liquids. The effects on the CHF are mainly discussed about the pressure in which the density ratio of the test fluid _v / _l varies from 6.24 × 10^–4 to 0.16 and of the ratio ofL/D _he =2 to 500, where an equivalent heated length,D _he , serves as a parameter connecting the clearance with the heated tube diameter. The experiment shows that the CHF obtained depends on the ratio ofL/D _he and then a generalized correlation can be derived predicting the CHF data well.

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Kritische Wärmestromdichte beim Sieden unter natürlicher Konvektion an gleichförmig beheizten Innenrohren vertikaler Ringrohre

Zusammenfassung Die kritische Wärmestromdichte (CHF) beim Sieden unter natürlicher Konvektion an gleichförmig beheizten Innenrohren senkrechter Ringrohre wurde experimentell untersucht, und zwar bei Drücken vonP=0,1 bis 3,1 MPa, Spaltweiten von 0,4 bis 4,0 mm, Durchmessern des beheizten Rohres von 5 bis 10,6 mm, Längen des Ringrohres vonL=58 bis 840 mm und mit drei Arten von Flüssigkeiten. Die Einflüsse auf die CHF werden hauptsächlich in dem Druckbereich diskutiert, bei welchem das Dichteverhältnis der Testflüssigkeit, _v / _l , zwischen 6,24 · 10^–4 und 0,16 variiert und das VerhältnisL/D _he von 2 bis 500 reicht. Die äquivalente beheizte LängeD _he stellt dabei einen Parameter dar, welcher die Spaltweite mit dem Durchmesser des Heizrohres in Beziehung bringt. Die Experimente belegen, daß die erhaltenen CHF-Werte vom VerhältnisL/D _he abhängen, woraus sich eine verallgemeinerte Beziehung herleiten läßt, welche die CHF-Daten gut vorauszuberechnen erlaubt.

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Nomenclature D _he equivalent heated length = {(D _o /D _i )² – 1} ·D _i = 4s/D _i · (1 +s/D _i ) - D _i outer diameter of a heated tube - D _o inner diameter of outertube - g gravitational acceleration - H _fg latent heat of evaporation - K constant (=0.16) - L length of the heated tube or surface in flow direction - P system pressure - q _co critical heat flux for saturated boiling - s clearance of annulus - Kutateladze number - _l density of saturated liquid - _v density of saturated vapor - surface tension     

Effect of subcooling on critical heat flux during pool boiling on a horizontal heated wire

Critical heat flux(CHF) is measured during pool boiling of water and R113 on a heated horizontal wire submerged in a subcooled liquid. Experiments are conducted over a pressure range from 0.1 to 3.0 MPa and subcooling up to 220 K. CHF data reveal that the CHF increases in a linear fashion with an increase in subcooling, and that the increment of the CHF with increasing subcooling becomes larger with increasing pressure. The characteristics of the CHF obtained differ from those of existing correlations at high pressures, although it is a similar tendency to them in that the CHF is proportional to the subcooling. A new correlation is derived by taking into account the effect of both the density ratio, ρ _L /ρ _V , and the Peclet number, Pe, and it succeeds in predicting the CHF data up to higher pressure and higher subcooling ranges, more effectively than previous studies using existing applicable ranges. Received on 23 July 1997     

Critical heat flux in forced convective boiling with a plane jet (Revised correlation for saturated condition)

 Critical heat flux (CHF) has been measured in saturated forced convective boiling with a wall jet on a rectangular heated surface of 40 and 80 mm in length and 20 mm in width. The jet velocity is varied from 3 to 15  m/s, and the system pressure is 0.1, 0.2, and 0.4 MPa for R113. It is found that the existing correlation for saturation condition can be applied to the CHF at high and low ρ_l/ρ_g values (e.g. water and R22), but hardly to the CHF at medium ρ_l/ρ_g values (e.g. R113 at 0.2 and 0.4 MPa). A revised correlation is proposed to predict most of the CHF data within an accuracy of ±25%. Received on 26 April 1999     

Free convective heat-transfer performance of a two-dimensional open thermosyphon with heat sources of cavity dotted along vertical wall

This paper is concerned with the heat-transfer characteristics in a vertical two-dimensional open thermosyphon whose heat sources are the heated cavities dotted along the vertical wall. Air is utilized for the measurement of heat transfer, while transformer oil for the observation of the flow patterns. Attention is particularly focussed on the effects of the depth of cavity and the clearance for main fluid-flow on the behavior of free convective heat transfer in the present open thermosyphon. Environmental temperature is maintained at 10°C, while temperature of the bottom-surface of cavity and the clearance of main fluid-flow are parametrically varied, as Rayleigh number ranging from 1.2×10¹ to 3.8×10⁶.It is found that the effect of the clearance on the heat-transfer characteristics in the two-dimensional open thermosyphon is unexpectedly large. Experimental results are finally given as plots of Nusselt number versus Rayleigh number. An experimental correlation is given for the Nusselt number as a function of the Rayleigh number and the clearance/length ratio of the open thermosyphon.

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Wärmeübertragungsverhalten bei freier Konvektion eines zweidimensionalen, offenen Thermosyphons mit längs der vertikalen Wand verteilten Hohlräume als Wärmequellen

Zusammenfassung Der Bericht befaßt sich mit dem Wärmeübertragungsverhalten in einem vertikalen, zweidimensionalen offenen Thermosyphon mit längs der vertikalen Wand verteilten, beheizten Hohlräumen als Wärmequellen. Zur Messung des Wärmeüberganges wird Luft, zur Strömungsbeobachtung Transformatorenöl verwendet. Besonderes Interesse gilt den Einflüssen der Hohlraumtiefe und der lichten Weite für den Hauptstrom auf das Verhalten des Wärmeüberganges bei freier Konvektion. Die Umgebungstemperatur wird auf 10°C gehalten, während die Hohlraumbodentemperatur und die lichte Weite für den Hauptstrom variiert werden mit Rayleigh-Zahlen zwischen 1.2×10¹ und 3.8×10⁶.Es wird festgestellt, daß der Einfluß der lichten Weite auf das Wärmeübertragungsverhalten unerwartet groß ist. Die experimentellen Ergebnisse werden in Diagrammen der Nusselt-Zahl über der Rayleigh-Zahl dargestellt. Ein Zusammenhang für die Nusselt-Zahl als Funktion von der Rayleigh-Zahl und dem Verhältnis von lichte Weite zu Länge wird gegeben.

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Nomenclature B distance between heated wall and opposing insulation wall,W+D - d _i diameter of inner tube - d ₀ diameter of outer heated tube - D depth of cavity along vertical wall, 0, 25, and 50 mm - g gravitational acceleration - H length of heated or un-heated wall, 100 mm - L length of thermosyphon, 500 mm for two-, 700 mm for three-, and 1100 mm for five-dotted heat sources - Nu _B Nusselt number based on B as reference length - Nu _x Nusselt number, defined in Eq. (1) - Pr Prandtl number, defined in Eq. (3) - q heat flux from heated wall - r equivalent heat-transfer radius - Ra _B Rayleigh number based on B as reference length - Ra _x Rayleigh number, defined in Eq. (2) - T _e temperature of entrance-fluid - T _w temperature of heated wall - T temperature difference between heated wall and entrance-fluid,T _w -T _e - W clearance for main fluid-flow - x reference length - X distance from bottom of thermosyphon Greek symbols coefficient of volumetric expansion - thermal diffusivity - thermal conductivity - kinematic viscosity     

A study of pulsations in a plasmatron with a self-stabilizing ARC

Shunting is one of the basic processes which determine the properties of the arc and heated gas stream in a vortex plasmatron. Much attention has, therefore, been given recently to the character of the shunting mechanism and of the associated pulsations. The relationship between fluctuations of the flux brightness at the plasmatron outlet and the arc voltage is described in [1]. Paper [2] contains data on the effect of gas consumption and current on shunting frequency; it also shows the possibility of coexistence of several anode spots in an argon arc. The authors of [3] investigated the effect of electrode channel and electrode material polarities on arc voltage pulsations. In the present study we obtain voltage, current, and brightness distribution functions and reproduce oscillograms of these quantities and also of the current through the anode cross sections. We use various methods to prove the coexistence of several anode spots and discuss the effect of the intrinsic magnetic field of the arc on its behavior.Notation I, U time-average values of the arc current and voltage - I_n time-average current through the n-th anode cross section - I_nt instantaneous value of the variable component of the current through the n-th anode cross section - Iz time-average current per unit length of the anode - I_t, U_t, B_t instantaneous values of the variable components of the current, voltage, and jet brightness - I_s, U_s, b_s instantaneous values of the current, voltage, and jet brightness - f _u,f _i,f _b voltage, current, and jet brightness distribution functions - G gas consumption (flow rate) through the arc chamber - d inside diameter of the anode - t time - z coordinate along the arc chamber axis (whose origin lies at the cathode end face) - v _z time-average frequency of visitation of a unit anode length by an arc spot - v _n time-average frequency of visitation of the n-th anode cross section by an arc spot - i, v distribution functions of the quantities I_z andv _z over the length of the anode - z_0i, z_0v values of z corresponding to the maxima of i andv - j current density in the arc column - H intensity of the magnetic self-field of the arc - s cross-sectional area of the arc column The authors are grateful to Yu. S, Dudnikov for his assistance.     

Measurements of heat transfer coefficients between turbulent liquid jets and a radially finned heated disk placed at the bottom of an open vertical circular cavity

 Experiments have been performed to assess the impact of an extended surface on the heat transfer enhancement for axisymmetric, turbulent liquid jet impingement on a heated round disk. The disk, with an array of integral radial fins mounted on its surface, is placed at the bottom of an open vertical circular cavity. Hydrodynamic and heat transfer data were obtained for a dielectric fluorocarbon liquid FC-77. For a fixed circular heater of diameter D=22.23 mm, several geometric parameters were tested: the nozzle diameter (4.42≤d≤9.27 mm), the confining wall diameter of the vertical cavity (22.23≤D _c≤30.16 mm), and the nozzle-to-heater spacing (0.5≤S/d≤5.0). The FC-77 flow rates varied from =0.2 to 11.0 l/min producing Reynolds numbers in the wide interval 700≤Re _d ≤44,000. For d=4.42 mm, the heat transfer response to the separation distance S/d was small but increased gradually with increasing nozzle diameter up to d=9.27 mm. The thermal resistance R _th increased with the confining wall diameter D _c and also with the nozzle diameter d. A minimum value of the thermal resistance of R _th,min=0.4 cm² K/W was attained for a combination of d=4.42 mm, D _c=22.23 mm, S/d=1, and =7.5 l/min. Based on a simplified heat transfer model, reasonable agreement was obtained between measured values of the thermal resistance and the R _th-predictions. The total fin effectiveness ɛ_f was shown to increase with increasing nozzle diameter, but was invariant with the flow rate (or the jet exit velocity). More than a three-fold heat transfer enhancement was realized through the addition of the array of integral radial fins on the heated round disk. Received on 30 August 2000 / Published online: 29 November 2001     

Critical heat flux in forced convective subcooled boiling with a plane wall jet (effect of subcooling on CHF)

Critical heat flux (CHF) during forced convective subcooled boiling with a plane jet has been yet made insufficient except for saturation condition when comparing CHF with impinging jet system including multiple jets. The present experiment has measured the CHF with plane jet on a rectangular heated surface of 40, 60, and 80 mm in length and 10 and 20 mm in width. Subcooled liquid being supplied through the plane jet with a different thickness of 1 and 2 mm, covers the heated surface where rigorous boiling takes place. The experiment varies a jet velocity from 3 to 15 m/s, a subcooling from 0 to 60 K, and system pressure at 0.1 MPa for water and at 1.5 to 3.0 MPa for R22. It is found that the existing correlation for saturation can be applied to the CHF of water, but hardly to the CHF of R22 in spite of saturation condition. After the effects of jet velocity and subcooling on the CHF can be elucidated, a new correlation including the effect of subcooling is proposed to predict most of the CHF data within an accuracy of ±20 percent. This correlation for saturated condition is found to interestingly agree with that theoretically derived by applying the Katto and Haramura criterion to this system. Received on 8 January 1997     

Influence of nanoparticle surface coating on pool boiling

The purpose of this work is to study the effects of nanostructured surface coatings on boiling heat transfer and CHF. Boiling experiments are performed on a 100 μm diameter platinum wire immersed in saturated water or pentane at 1 bar. Nanostructured surface coating is obtained by deposition of charged γ-Fe₂O₃ nanoparticles (average diameter of 10 nm) on the platinum wire. Two different processes are compared: vigorous boiling and electrophoresis.The deposition of nanoparticles onto the heated surface induces a significant increase of the boiling critical heat flux (CHF) related to the increase of wettability. It also induces a decrease of the heat transfer coefficient when the wire is entirely covered with nanoparticles. The critical heat flux enhancement depends on the wettability of the fluid compared with the bare heater. Different physical mechanisms are also studied to explain the evolution of the characteristic parameters of the boiling on nanostructured surfaces.     

Experimental and numerical investigations on PDE performance augmentation by means of an ejector

To improve the performance of pulse detonation engines, a 48 cm long cylindrical combustion chamber of 5cm internal diameter (i.d.) is fitted with an ejector of constant section. The role of the ejector is (i) to provide partial confinement of the detonation products escaping from the chamber and (ii) to suck in fresh air and then to increase the mass ejected compared to the ejection of burned gases alone.The combustion chamber is fully filled with a stoichiometric ethylene/oxygen mixture at ambient conditions. Three parameters of the ejector are varied: the i.d. D, the length L, and the position d relative to the thrust wall of the combustion chamber. For various configurations, the specific impulse (I _sp) is determined in single shot experiments. The maximum operating frequency (f _max) and the maximum thrust are then deduced. I _sp is measured by means of the ballistic pendulum method, and f _max is derived from the pressure signal recorded on the combustion chamber thrust wall.The addition of an ejector increases the specific impulse up to 60% in the best configuration tested, from 164s without ejector to 260s with ejector. The specific impulse can be represented by a single curve using suitable dimensionless parameters. The thrust results for the main ejector studied (D = 80mm) indicate an optimal (L, d) configuration that provides a 28% thrust gain. For the same ejector, f _max remains constant and equal to the frequency obtained without ejector in a large range of (L, d) values, before decreasing.Two-dimensional unsteady numerical computations agree reasonably with the experiments, slightly overestimating the experimental values. The results indicate that 80% of the I _sp gain comes from the action of the expanding detonation products on the annular end surface of the combustion chamber, governed by the tube wall thickness.This paper was based on the work that was presented at the 19th International Colloquium on the Dynamics of Explosions and Reactive Systems, Hakone, Japan, July 27–August 1, 2003 PACS 47.40.Rs     

Spacing effects on hydrodynamics of heave plates on offshore structures

The nonlinear viscous flow problem associated with a heaving vertical cylinder with two heave plates in the form of two circular disks attached is solved using a finite difference method. Numerical experiments are carried out to investigate the spanwise length effects on the hydrodynamic properties, such as added mass and damping coefficients. Over a Keulegan–Carpenter (KC) number range from 0.1 to 0.6 at frequency parameter (β=7.869×10⁷), calculations indicate that a KC-dependent critical value of spanwise length L/D_d (L—the distance between the disks, and D_d—the diameter of the disks) exists. A significant influence of L/D_d on the vortex shedding patterns around the disks and the hydrodynamic coefficients is revealed when L/D_d is smaller than the critical value due to strong interaction between vortices of different disks. Beyond that limit, however, both added mass and damping coefficients are shown to be rather stable, indicating the independence of the vortex shedding processes of different disks.     

Measurement and scaling analysis of critical energy for direct initiation of gaseous detonations

In this paper, the critical energies required for direct initiation of spherical detonations in four gaseous fuels (C₂H₂, C₂H₄, C₃H₈ and H₂)–oxygen mixtures at different initial pressures, equivalence ratios and with different amounts of argon dilution are reported. Using these data, a scaling analysis is performed based on two main parameters of the problem: the explosion length R _o that characterizes the blast wave and a characteristic chemical length that characterizes the detonation. For all the undiluted mixtures considered in this study, it is found that the relationship is closely given by R_o ? 26 l{R_{\rm o} \approx 26 \lambda} , where λ is the characteristic detonation cell size of the explosive mixture. While for C₂H₂–2.5O₂ mixtures highly diluted with argon, in which cellular instabilities are shown to play a minor role on the detonation propagation, the proportionality factor increases to 37.3, 47 and 54.8 for 50, 65 and 70% argon dilution, respectively. Using the ZND induction length Δ _I as the characteristic chemical length scale for argon diluted or ‘stable’ mixtures, the explosion length is also found to scale adequately with R_o ? 2320 D_I{R_{\rm o} \approx 2320 \Delta_I} .     

Empirical Measures of Wettability in Porous Media and the Relationship between Them Derived From Pore-Scale Modelling

The wettability of a crude oil/brine/rock system is of central importance in determining the oil recovery efficiency of water displacement processes in oil reservoirs. Wettability of a rock sample has traditionally been measured using one of two experimental techniques, viz. the United States Bureau of Mines and Amott tests. The former gives the USBM index, I _USBM, and the latter yields the Amott–Harvey index, I _AH. As there is no well-established theoretical basis for either test, any relationship between the two indices remains unclear.Analytical relationships between I _AH and I _USBM for mixed-wet and fractionally-wet media have been based on a number of simplifying assumptions relating to the underlying pore-scale displacement mechanisms. This simple approach provides some guidelines regarding the influence of the distribution of oil-wet surfaces within the porous medium on I _AH and I _USBM. More detailed insight into the relationship between I _AH and I _USBM is provided by modelling the pore-scale displacement processes in a network of interconnected pores. The effects of pore size distribution, interconnectivity, displacement mechanisms, distribution of volume and of oil-wet pores within the pore space have all been investigated by means of the network model.The results of these analytical calculations and network simulations show that I _AH and I _USBM need not be identical. Moreover, the calculated indices and the relationship between them suggest explanations for some of the trends that appear in experimental data when both I _USBM and I _AH have been reported in the literature for tests with comparable fluids and solids. Such calculations should help with the design of more informative wettability tests in the future.     

High-resolution imaging of turbulence structures in jet flames and non-reacting jets with laser Rayleigh scattering

A comparative study of the length scales and morphology of dissipation fields in turbulent jet flames and non-reacting jets provides a quantitative analysis of the effects of heat release on the fine-scale structure of turbulent mixing. Planar laser Rayleigh scattering is used for highly resolved measurements of the thermal and scalar dissipation in the near fields of CH₄/H₂/N₂ jet flames (Re _d  = 15,200 and 22,800) and non-reacting propane jets (Re _d  = 7,200–21,700), respectively. Heat release increases the dissipation cutoff length scales in the reaction zone of the flames such that they are significantly larger than the cutoff scales of non-reacting jets with comparable jet exit Reynolds numbers. Fine-scale anisotropy is enhanced in the reaction zone. At x/d = 10, the peaks of the dissipation angle PDFs in the Re _d  = 15,200 and 22,800 jet flames exceed those of non-reacting jets with corresponding jet exit Reynolds numbers by factors of 2.3 and 1.8, respectively. Heat release significantly reduces the dissipation layer curvature in the reaction zone and in the low-temperature periphery of the jet flames. These results suggest that the reaction zone shields the outer regions of the jet flame from the highly turbulent flow closer to the jet axis.     

Effects of ultrasonic vibration on subcooled pool boiling critical heat flux

The effects of ultrasonic vibration on critical heat flux (CHF) have been experimentally investigated under natural convection condition. Flat bakelite plates coated with thin copper layer and distilled water are used as heated specimens and working fluid, respectively. Measurements of CHF on flat heated surface were made with and without ultrasonic vibration applied to working fluid. An inclination angle of the heated surface and water subcooling are varied as well. Examined water subcoolings are 5°C, 20°C, 40°C and the angles are 0°, 10°, 20°, 45°, 90°, 180°. The measurements show that ultrasonic wave applied to water enhances CHF and its extent is dependent upon inclination angle as well as water subcooling. The rate of increase in CHF increases with an increase in water subcooling while it decreases with an increase in inclination angle. Visual observation shows that the cause of CHF augmentation is closely related with the dynamic behaviour of bubble generation and departure in acoustic field.     

Effects of the corner radius on the near wake of a square prism

The near wake of square cylinders with different corner radii was experimentally studied based on particle imaging velocimetry (PIV), laser doppler anemometry (LDA) and hotwire measurements. Four bluff bodies, i.e., r/d=0 (square cylinder), 0.157, 0.236, 0.5 (circular cylinder), where r is corner radius and d is the characteristic dimension of the bluff bodies, were examined. A conditional sampling technique was developed to obtain the phase-averaged PIV data in order to characterize quantitatively the effect of corner radii on the near-wake flow structure. The results show that, as r/d increases from 0 to 0.5, the maximum strength of shed vortices attenuates, the circulation associated with the vortices decreases progressively by 50%, the Strouhal number, St, increases by about 60%, the convection velocity of the vortices increases along with the widening of the wake width by about 25%, the vortex formation length and the wake closure length almost double in size. Meanwhile, both the vortex wavelength, λ _x , and the lateral spacing, λ _y , decrease as r/d increases, but the ratio of λ _y to λ _x is approximately 0.29, irrespective of r/d, which is close to the theoretical value of 0.281 for a stable Karman vortex street. The decrease in wavelength is probably responsible for the change in the flow structure from the approximately circular-shaped vortex at r/d=0 to the laterally stretched vortex at r/d=0.5. The leading edge corner radius is more important than the trailing one in influencing the near wake structure since it determines to a great extent the behavior of the streamlines, the separation angle and the base pressure. It is further found that the ratio of the mean drag coefficient to the total shed circulation, C _d/Γ₀, approaches a constant, about 0.25 for different bluff bodies in the subcritical flow regime. The streamwise evolution of vortices and the streamwise fluctuating velocity along the centerline for rounded cylinders are also discussed.     

A Sharp-Interface Limit for a Two-Well Problem in Geometrically Linear Elasticity

In the theory of solid-solid phase transitions the deformation of an elastic body is determined via a functional containing a nonconvex energy density and a singular perturbation. We study Frame indifference, within a linearized setting, requires that W depends only on the symmetric part of ∇u. The potential W is nonnegative and vanishes on two wells, i.e., for d = 2, on two lines in the space of matrices. We determine, for d = 2, the Gamma limit I₀ = Γ− lim _ɛ→0 I_ɛ. The limit I₀[u] is finite only for deformations u that fulfill W(∇u)=0 almost everywhere and have sharp interfaces where ∇u has jumps. For these u, I₀[u] equals the line integral over the interfaces of a surface energy density.     

Detonability of THDCPD-exo–air mixtures

In the frame of industrial risk and propulsive application, the detonability study of JP10–air mixtures was performed. The simulation and measurements of detonation parameters were performed for THDCPD-exo/air mixtures at various initial pressure (1 bar < P ₀ < 3 bar) and equivalence ratio (0.8 < Φ < 1.6) in a heated tube (T ₀ ~ 375 K). Numerical simulations of the detonation were performed with the STANJAN code and a detailed kinetic scheme of the combustion of THDCPD. The experimental study deals with the measurements of detonation velocity and cell size λ. The measured velocity is in a good agreement with the calculated theoretical values. The cell size measurements show a minimum value for Φ ~ 1.2 at every level of initial pressure studied and the calculated induction length L _i corresponds to cell size value with a coefficient k = λ/L _i = 24 at P ₀ = 1 bar. Based on the comparison between the results obtained during this study and those available in the literature on the critical initiation energy E _c, critical tube diameter d _c and deflagration to detonation transition length L _DDT, we can conclude that the detonability of THDCPD–air mixtures corresponds to that of hydrocarbon–air mixtures.

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This paper is based on the work presented at the 33rd International Pyrotechnics Seminar, IPS 2006, Fort Collins, July 16–21, 2006.     

Characterization of material inhomogeneity by stationary values of strain energy density

The strain energy density theory is applied to analyze the fracture instability of a mechanical system whose behavior is governed by the interaction of geometry, load and material inhomogeneity. This is accomplished by obtaining the location of the global and local relative minima of the strain energy density function dW/dV denoted, respectively, by [(dW/dV)_min]_g and [(dW/dV)_min]_¢l. The former refers to a fixed global coordinate system for the entire solid while the latter corresponds to local coordinate systems referred to each material point. An unique length parameter “ℓ” representing the distance between [(dW/dV)_min^max]_g and [(dW/dV)_min^max]_ℓ can thus be found and serves as a measure of the degree of system instability tending toward failure by fracture.Numerical results are obtained and displayed graphically for the case of a solid containing an inclusion of dissimilar material. The changes that take place in material inhomogeneity, loading type and physical dimensions of the solid and inclusion are reflected through ℓ. The method suggests the compatibility of ℓ for each member of a multi-component structure in order to avoid premature failure of a single member.     

Status of prediction methods for critical heat fluxes in mini and microchannels

Saturated critical heat flux (CHF) is an important issue during flow boiling in mini and microchannels. To determine the best prediction method available in the literature, 2996 data points from 19 different laboratories have been collected since 1958. The database includes nine different fluids (R-134a, R-245fa, R-236fa, R-123, R-32, R-113, nitrogen, CO₂ and water) for a wide range of experimental conditions. This database has been compared to 6 different correlations and 1 theoretically based model. For predicting the non-aqueous fluids, the theoretical model by Revellin and Thome [Revellin, R., Thome, J.R., 2008. A theoretical model for the prediction of the critical heat flux in heated microchannels. Int. J. Heat Mass Transfer 51, 1216–1225] is the best method. It predicts 86% of the CHF data for non-aqueous fluids within a 30% error band. The data for water are best predicted by the correlation by Zhang et al. [Zhang, W., Hibiki, T., Mishima, K., Mi, Y., 2006. Correlation of critical heat flux for flow boiling of water in minichannels. Int. J. Heat Mass Transfer 49, 1058–1072]. This method predicts 83% of the CHF data for water within a 30% error band. Some suggestions have also been proposed in this paper for the future studies.