Opposing Flow Conditions Research Articles

Dynamic tracking of a magnetic micro-roller using ultrasound phase analysis

Microrobots (MRs) have attracted significant interest for their potentialities in diagnosis and non-invasive intervention in hard-to-reach body areas. Fine control of biomedical MRs requires real-time feedback on their position and configuration. Ultrasound (US) imaging stands as a mature and advantageous technology for MRs tracking, but it suffers from disturbances due to low contrast resolution. To overcome these limitations and make US imaging suitable for monitoring and tracking MRs, we propose a US contrast enhancement mechanism for MR visualization in echogenic backgrounds (e.g., tissue). Our technique exploits the specific acoustic phase modulation produced by the MR characteristic motions. By applying this principle, we performed real-time visualization and position tracking of a magnetic MR rolling on a lumen boundary, both in static flow and opposing flow conditions, with an average error of 0.25 body-lengths. Overall, the reported results unveil countless possibilities to exploit the proposed approach as a robust feedback strategy for monitoring and tracking biomedical MRs in-vivo.

On the effects of opposed flow conditions on non-buoyant flames spreading over polyethylene-coated wires – Part II: Soot oxidation quenching and smoke release

On the effects of opposed flow conditions on non-buoyant flames spreading over polyethylene-coated wires – Part II: Soot oxidation quenching and smoke release

On the effects of opposed flow conditions on non-buoyant flames spreading over polyethylene-coated wires – Part II: Soot oxidation quenching and smoke release

Smoke release in the limited volume of a spaceship poses a major threat to the life of astronauts in long range exploration missions. If the absence of buoyant flows fundamentally affects combustion mechanisms, the possibility of atmospheric design in spacecraft environment provides a new leverage, not usually available on Earth. Investigating a non-buoyant flame spreading over the polyethylene coating of an electrical wire in an opposed laminar flow, the previous paper highlighted how flow conditions, namely oxygen content, flow velocity, and ambient pressure, affected spread rate and soot formation rate. The implementation of the Broadband Modulated Absorption/Emission (B-MAE) technique provided mappings of soot temperature and volume fraction in the spreading flames during parabolic flights. In this second paper, the link between these microscopic observations and new macroscopic findings regarding the influence of flow conditions on the smoke production of a flame in the same configuration is presented. Taking into account other requirements of space exploration, atmospheric conditions below normoxic values present a clear interest from a fire safety perspective. In the process, a threshold temperature at which soot oxidation reactions are frozen is identified. The value of 1400 K brought forward conforms with past measurements performed at normal gravity, while discrepancies with previous microgravity measurements are addressed. Given the broad capability of human lungs to adapt to various conditions, the overall mapping of smoke production as a function of flow conditions is a valuable tool for atmospheric design considerations.

On the effects of opposed flow conditions on non-buoyant flames spreading over polyethylene-coated wires – Part I: Spread rate and soot production

In microgravity, the extended time scales associated with the absence of buoyancy lead to peculiar flame features that are likely to affect the risk associated with flame spread in the case of a spacecraft fire. Investigating a non-buoyant flame spreading over the polyethylene coating of an electrical wire in an opposed laminar flow, recent studies especially evidenced and quantified the major role of soot in the radiative heat transfer, which affects local quenching at the flame tip as well as heat feedback from the flame to the coating. Consequently, the control of soot production in such a flame needs to be explored. In the present paper, the role of basic flow features, i.e. oxygen content, flow velocity, and ambient pressure, is documented. Conducted in parabolic flights, a set of 142 experiments spreads over 91 flow conditions, with oxygen content ranging from 18% to 21%, flow velocity kept between 100 mm s−1 and 200 mm s−1, and pressure ranging from 51 kPa to 142 kPa. The implementation of the Broadband Modulated Absorption/Emission (B-MAE) technique allows the fields of soot temperature and volume fraction to be measured within the spreading flames. The flame spread rate is thus shown to be an increasing function of oxygen content, but is independent of flow velocity and pressure. Concomitantly, both oxygen content and flow velocity affect soot production residence time, while pressure has a marginal impact on it. Maximum soot volume fraction is a function of all three parameters. Complementing these results with a scaling analysis, soot production rate is third-order in pressure, and very sensitive to oxygen content. An increase in flow velocity promotes two competitive processes with respect to maximum soot volume fraction, i.e. a reduction in residence time and an increase in flame temperature. A numerical model supports the experimental finding that the latter phenomenon prevails, hence that maximum soot volume fraction increases with flow velocity. These conclusions will serve as basis for upcoming quenching and radiative heat feedback analysis.

Analysis of mixed convection heat transfer of nanofluids in a minichannel for aiding and opposing flow conditions

In this study, mixed convection heat transfer characteristics of nanofluid flow in a circular minichannel were investigated experimentally. The effects of the particle volume ratio (0, 0.25, and 0.75%) and Reynolds number (200-60) on heat transfer by mixed convection were investigated for aiding and opposing flow conditions. Water and water based SiO2 nanofluids were used as working fluid in a minichannel with a diameter of 1.9 mm, and constant heat flux was applied to the outer surface of the minichannel. Temperature-dependent thermophysical properties such as thermal conductivity and viscosity, obtained by experimental measurements, were used in heat transfer calculations. Pressure based numerical computations for all experimental cases were also made by using single phase approach. The results were analyzed separately for aiding flow condition in which secondary flows originating from natural convection are in the same direction with the forced flow, and for opposing flow condition in which secondary flows are in the opposite direction with forced flow. For detailed analysis of mixed convection heat transfer, temperature contours and velocity profiles were obtained by the numerical computations which were compared and validated with the experimental results. According to the data obtained, it was determined that the addition of nanoparticles into the pure water increased the Nusselt number by around 21–64% for the aiding flow, and 18–58% for the opposing flow. The heat transfer in the aiding flow was observed to be minimum 4% and maximum 16% higher in comparison with the opposing flow condition. It was concluded that the direction of secondary flows significantly affects heat transfer.

Downward flame spread along a single pine needle: Numerical modelling

In this work downward flame spread over single pine needle of Pinus Sibirica is studied. Pine needles are thin cellulosic charring combustible forest fuel elements. Idealising pine needles to thin cylinders, a 2D axisymmetric numerical model is developed accounting for char formation and char oxidation to investigate the important mechanisms which control the downward spread of flame over a pine needle in normal gravity, atmospheric condition and at various opposed flow conditions. Simultaneous formation of char and pyrolysate during the pyrolysis process was found to significantly reduce the flame spread rate over thin fuel. Presence of char resulted in change in distribution of fuel vapour mass flux above the fuel surface which led to decrease in forward heat feedback to the fuel and hence the flame spread rate. This mechanism is different from char acting as a thermal barrier to heat transfer from the flame in case of thick fuel. Char oxidation had no influence on flame spread rate as char oxidation was found to occur only after passage of flame with the availability of surrounding oxygen diffusing through the hot plume of combustion products. Char oxidation was primarily controlled by oxygen diffusion rate to the charred fuel surface. The flame spread data for quiescent flame spread, and the blow off opposed flow velocity was used to calibrate gas phase kinetics and pyrolysis kinetics. The model predicted flame spread rate variation with opposed flow velocity quite well. The predicted spatial distribution of temperature and species concentration also compared very well with the experimentally determined flame structure.

Assisting and Opposing Combined Convective Heat Transfer and Nanofluids Flows Over a Vertical Forward Facing Step

Numerical simulations of two-dimensional (2D) laminar mixed convection heat transfer and nanofluids flows over forward facing step (FFS) in a vertical channel are numerically carried out. The continuity, momentum, and energy equations were solved by means of a finite volume method (FVM). The wall downstream of the step was maintained at a uniform wall heat flux, while the straight wall that forms the other side of the channel was maintained at constant temperature equivalent to the inlet fluid temperature. The upstream walls for the FFS were considered as adiabatic surfaces. The buoyancy assisting and buoyancy opposing flow conditions are investigated. Four different types of nanoparticles, Al2O3, CuO, SiO2, and ZnO with different volumes' fractions in the range of 1–4% and different nanoparticle diameters in the range of 25–80 nm, are dispersed in the base fluid (water) are used. In this study, several parameters, such as different Reynolds numbers in the range of 100 < Re < 900, and different heat fluxes in the range of 500 ≤ qw ≤ 4500 W/m2, and different step heights in the range of 3 ≤ S ≤ 5.8 mm, are investigated to identify their effects on the heat transfer and fluid flow characteristics. The numerical results indicate that the nanofluid with SiO2 has the highest Nusselt number compared with other nanofluids. The recirculation region and the Nusselt number increase as the step height, Reynolds number, and the volume fraction increase, and it decreases as the nanoparticle diameter increases. This study has revealed that the assisting flow has higher Nusselt number than opposing flow.

Extinction limits of spreading flames over wires in microgravity

Tests with flames spreading over wires in microgravity were performed at external opposed flow conditions from 60 to 200mm/s to examine the influence of flow velocity on the extinction limit. In the experiments, low density polyethylene insulated Nickel–chrome and Copper wire samples were used. The experiments were conducted both in normal gravity and microgravity attained by parabolic flights. The experimental results are discussed based on non-dimensional numbers developed by Takahashi et al. This study added the heat conduction through the wire insulation to the inner core to their model, which is considered as a heat loss term arising due to the presence of the inner core. Further the effect of curvature of the sample is taken into consideration. From the results of the experiments in normal gravity, there was a low oxygen limit for Copper wire at around 125mm/s. However there was no low oxygen limit for Nickel–chrome wire; the limits of the oxygen concentration decreased monotonically with decreases in the external flow velocity. The non-dimensional number implies that the decrease in inner core temperature around the preheat zone occurring with Copper wire plays a role in the change in low oxygen limits. Further, in the external flow velocity range investigated here, there was no low oxygen limit in microgravity with the Copper wire. This might arise due to the changes in the flame shapes by gravity. The heat input from flame to the wire is supposed to increase in microgravity because the wire is surrounded by flame due to inhibited natural convection. This increased heat input might help the increase of the inner core temperature around the preheat zone. For this reason, there is a possibility that the low oxygen limit with Copper wire in microgravity exists at lower flow velocity than that in normal gravity.

Study on unsteady molten insulation volume change during flame spreading over wire insulation in microgravity

Tests with flames spreading over wire insulation in microgravity were performed at varying external opposed flow conditions to examine the influence of flow velocity in the time dependent volume change of molten insulation. In the experiments, low density polyethylene insulated Nickel–chrome wire specimens were used and the oxygen concentration was fixed at 30% (N2 balanced). The results show that the time dependent changes in molten insulation volume are related to the opposed flow velocity. Further, as opposed flow velocity increases, the volume change rate decreases monotonically. By subtracting the volume change rate from the volume supply rate from the solid part to the molten part, which is calculated by multiplying the rate of spreading of molten insulation at the leading edge by the cross sectional area of the insulation, a pyrolysis volume rate for the polyethylene was established. The pyrolysis volume rate is defined as the amount of consumed molten insulation volume per unit time. After these calculations, it was found that the pyrolysis volume rate increases monotonically with increases in the opposed flow velocity. Further, numerical calculations of time dependent volume change in the molten insulation at different flow velocities were made. The numerical results show good agreement with the experimental results of the molten insulation volume change during the 0–4.5s of microgravity measured here. By using the numerical calculations for this initial short period, the time dependent volume change in molten insulation during longer-term microgravity is predicted. The calculated results show that the volume finally reaches a steady state value in flow velocities of 10–250mm/s investigated here. These results provide insight into the mechanism of flame spreading over wire insulation, especially the unsteadiness of the flame in flame spreading events.

HEAT AND MASS TRANSFER IN STAGNATION-POINT FLOW OF A POLAR FLUID TOWARDS A STRETCHING SURFACE IN POROUS MEDIA IN THE PRESENCE OF SORET, DUFOUR AND CHEMICAL REACTION EFFECTS

This work is focused on the numerical solution of steady boundary-layer stagnation-point flow of a polar fluid towards a stretching surface embedded in porous media in the presence of the effects of Soret and Dufour numbers and first-order homogeneous chemical reaction. The governing boundary-layer equations of the problem are formulated and transformed into a self-similar form. The obtained equations are solved numerically by an efficient, iterative, tri-diagonal, implicit finite-difference method. Both assisting and opposing flow conditions are considered. Comparisons of the present numerical results with previously published work under limiting cases are performed and found to be in excellent agreement. Representative results for the fluid velocity, angular velocity, temperature, and solute concentration profiles as well as the local heat and mass transfer rates for various values of the physical parameters are displayed in both graphical and tabular forms.

AIDING AND OPPOSING MIXED CONVECTION FROM AN ARRAY OF CIRCULAR CYLINDERS IN A SATURATED POROUS MEDIUM

Numerical solutions are presented for mixed convection from an array of circular cylinders embed in a saturated porous medium. The cylinders are at constant temperature(isothermal) and arranged in a staggered tube bank. Both aiding and opposing flow conditions are considered. Numerical calculations using finite difference method with body-fitted coordinates have covered a wide range of governing parameters(i.e.,10 ،ـ Re ،ـ 100, 0 ،ـ Gr ،ـ 400 and Pr = 0.7). Results are presented for streamline, isotherms and the local and the average Nusselt number at different values of the governing parameters. The present results are compared with previous theoretical results and show good agreement

Opposing Flow Turbulent Mixed-Convection Heat Transfer in a Vertical Flat Channel with One-Sided Heating

The experimental investigation of heat transfer in a vertical flat channel with one-sided heating for turbulent mixed-convection opposing flow conditions have been performed in the region of Re = 4·103−4·104 and Grq = 1.7·108−1.4·1010. The data was analyzed using several buoyancy parameters and was compared with the results obtained by other authors in the vertical tubes.

Mixed convection along a vertical cone for fluids of any Prandtl number: case of constant wall temperature

An analysis of steady laminar mixed convection boundary layer flow along a vertical cone of constant wall temperature is presented. A mixed convection parameter ξ, as proposed by Lin and Chen, is used to serve as a controlling parameter that determines the relative importance of the forced and the free convection flows. New coordinates and dependent variables are then defined in terms of ξ, so that the transformed non‐similar boundary layer equations give computationally efficient numerical solutions which are valid over the entire range of mixed convection flow from the forced convection limit to the free convection limit for fluids of any Prandtl number. The effects of the mixed convection parameter ξ and the Prandtl number Pr on the velocity and temperature profiles as well as on the skin friction and heat transfer coefficients are shown for both cases of buoyancy assisting and buoyancy opposing flow conditions.

Adverse and favorable mixed convection heat transfer in a two-side heated square channel

Abstract Experimental heat transfer measurements and analysis for mixed convection in a vertical square channel are presented. Water flow directions are selected such that buoyancy assists or opposes the bulk flow pressure gradient. Unlike most previous experiments with symmetrically heated circular tubes, the present configuration uses an asymmetric heating condition (two sides heated and two sides insulated) and shows significant increase in the Nusselt number for both assisted and opposed flow conditions. Observed heat transfer coefficient distributions are different from the symmetrically heated channels; and this difference in heat transfer coefficient is attributed to the formation of buoyancy driven large-scale flow structures. In general, opposed flow shows higher heat transfer coefficients, and the Nusselt number ratio is observed to increase as Gr/Re or Gr/Re 2 ratios increase for both assisted and opposed flow conditions. A correlation based on the buoyancy parameter predicts the heat transfer pattern well in both assisted and opposed mixed convection. The range of Reynolds numbers discussed (Re=400–10,000) is of importance for direct numerical simulations and the details provided here can serve as the benchmark data required for complicated buoyancy affected turbulence simulations.

New correlations for mixed turbulent natural and forced convection heat transfer in vertical tubes

This work deals with mixed natural and forced turbulent convection heat transfer in vertical tubes. It summarizes the experimental results available in literature for both aiding and opposing flow conditions and presents own experimental results that concentrated on the influence of length-to-diameter ratio and of heat and mass flux directions on heat transfer in vertical tubes. Finally, it presents two new empirical correlations that represent experimental results better than all existing correlations in literature, since they take into account the laminarization of turbulent flow that occurs during aiding mixed convection. They predict experimental results from literature and from this work with an accuracy of better than ± 20%.

Mixed convection heat transfer from a vertical plate to non-Newtonian fluids

The nonsimilar boundary-layer analysis of steady laminar mixed-convection heat transfer between a vertical plate and non-Newtonian fluids is extended and unified. A mixed-convection parameter ζ is proposed to replace the conventional Richardson number, Gr/Re 2/(2− n) and to serve as a controlling parameter that determines the relative importance of the forced and the free convection. The value of mixed-convection parameter lies between 0 and 1. In addition, the power-law model is used for nonNewtonian fluids with exponent n < 1 for pseudoplastics; n = 1 for Newtonian fluids; and n > 1 for dilatant fluids. Furthermore, the coordinates and dependent variables are transformed to yield computationally efficient numerical solutions that are valid over the entire range of mixed convection, from the pure forced-convection limit to the pure free-convection limit, and the whole domain of non-Newtonian fluids, from pseudoplastics to dilatant fluids. The effects of the mixed-convection parameter, the power-law viscosity index, and the generalized Prandtl number on the velocity profiles, the temperature profiles, as well as on the wall skin friction and heat transfer rate are clearly illustrated for both cases of buoyancy assisting and opposing flow conditions.

Convective heat transfer from a rotating or nonrotating axisymmetric body

An analysis of steady laminar mixed-convection heat transfer from a rotating or nonrotating axisymmetric body is presented. A mixed-convection parameter is proposed to serve as a controlling parameter that determines the relative importance of the forced and the free convection. In addition, a rotation parameter is introduced to indicate the relative contributions of the flow forced convection and the rotational forced convection. The values of both these two parameters lie between 0 and 1. Furthermore, the coordinates and dependent variables are transformed to yield computationally efficient numerical solutions that are valid over the entire range of mixed convection from the forced-convection limit (rotating or nonrotating bodies) to the pure free-convection limit (non-rotating bodies) and the entire regime of forced convection from the pure flow forced-convection limit (nonrotating bodies) to pure rotational forced-convection limit (rotating bodies). The effects of mixed-convection intensity, body rotation, fluid suction or injection, and fluid Prandtl number on the velocity profiles, the temperature profiles, the skin-friction parameter, and heat transfer parameter are clearly illustrated for both cases of buoyancy assisting and opposing flow conditions.

Laminar mixed convection in a duct with a backward-facing step: the effects of inclination angle and Prandtl number

Mixed convective heat transfer results for two-dimensional laminar flow in an inclined duct with a backward-facing step are presented for both the buoyancy assisting and the buoyancy opposing flow conditions. The wall downstream of the step is maintained at a uniform heat flux, while the straight wall that forms the other side of the duct is maintained at a constant temperature equivalent to the inlet fluid temperature. The wall upstream of the step and the backward-facing step are considered as adiabatic surfaces. The inlet flow is fully developed and is at a uniform temperature. The effects of the inclination angle and Prandtl number on the velocity and temperature distributions are reported.

Mixed convection from a horizontal plate to fluids of any Prandtl number

Laminar mixed convection over a horizontal plate with uniform wall temperature or uniform wall heat flux is analyzed by introducing proper buoyancy parameters and transformation variables for fluids of any Prandtl number between 0.001 and 10,000. Both cases of buoyancy assisting and opposing flow conditions are investigated. For the buoyancy-assisting case, the obtained numerical results are very accurate over the entire range of mixed convection intensity from pure forced convection limit to pure free convection limit. For the buoyancy-opposing case, solutions are obtained from the forced convection limit to the point of breakdown.

Mixed convection on vertical plate for fluids of any prandtl number

A mixed convection parameterζ=(σRa)1/4/(ωRe)1/2, withσ=Pr/(1+Pr) andω=Pr/(1 +Pr)1/2, is proposed to replace the conventional Richardson number, Gr/Re2, for combined forced and free convection flow on an isothermal vertical plate. This parameter can readily be reduced to the controlling parameters for the relative importance of the forced and the free convection,Ra1/4/(Re1/2Pr1/3) forPr ≫ 1, and (RaPr)1/2/(RePr1/2 forPr ≪ 1. Furthermore, new coordinates and dependent variables are properly defined in terms ofζ, so that the transformed nonsimilar boundary-layer equations give numerical solutions that are uniformly valid over the entire range of mixed convection intensity from forced convection limit to free convection limit for fluids of any Prandtl number from 0.001 to 10,000. The effects of mixed convection intensity and the Prandtl number on the velocity profiles, the temperature profiles, the wall friction, and the heat transfer rate are illustrated for both cases of buoyancy assisting and opposing flow conditions.