Relative Cavity Length Research Articles

Design of the Microchannels Heat Sink to Augment the Thermal Performance Using Different Type of Cavity with or without Wavy Channel

The development of innovative micro sinks design stems from the urge to perform efficient cooling at a cost of low pressure drop for more miniaturized electronic equipment. The three-dimensional investigations have been carried out with disruptive structures in rectangular microchannels. Microchannel heat sinks with triangular cavity, trapezoidal cavity, wavy plane channel, wavy triangular cavity, and wavy trapezoidal cavity have been considered. The performance variations are evaluated based on the average Nusselt number, friction factor, and thermal performance within the range of Reynolds numbers from 141 to 498. The highest thermal performance is achieved by wavy trapezoidal cavity among all channels. The parametric geometrical studies have also been carried out by varying relative geometric parameters such as relative amplitude, relative cavity length, and relative cavity width. The highest heat transfer enhancement equal to 2.33, for the same channel has been achieved with the combination of relative amplitude; relative cavity length and relative cavity width is equal to 0.75, 0.055, and 0.75, respectively at Reynolds number equal to 498. The wavy shapes give better mixing by creating of additional vortices or dean vortices to increase heat transfer with minimum pressure drop.

Study of Two-Phase Flow Behavior around Chute Aerator Using OpenFOAM

Aeration devices are installed on chute spillways to prevent cavitation damage in high-velocity flows. Large quantities of air-entrained characterize bottom aerators along with the jet interfaces and a strong de-aeration process near the impact of the water jet with the spillway bottom. Appropriate prediction of the air entrainment process, flow characteristics, and two-phase flow pattern at the aerator would contribute to reliable spillway operation. The mathematical formulation of two-phase flow at an aerator remains a challenging issue for spillway design due to its complexities. In the present study, 2D numerical simulations are performed to predict the distribution of air concentrations, flow velocity, turbulent intensity, and water surface profile along the chute aerator using open-source OpenFOAM software and RNG k-ε turbulent model. The correlation coefficients obtained between the numerical and experimental results indicate that a proper agreement exists between the relative cavity length and velocity profile results. The research results show that the turbulent intensity of flow passing over the aerator ramp is significantly increases. It means that the ramp acts as a turbulent generator on the spillway chute. The maximum value of turbulence intensity occurs at the 6-8% of flow depth(h) from the tip of the ramp, and it is raised as the Froude number increases. It is concluded that the intake air flow rate and air-entrainment coefficient (𝛽) are linearly increased concerning the Froude number.

Fin performance of 3-D aerator devices with backward lateral deflectors

In the present work, a 3-D aerator device with backward lateral deflectors, called BLD-3-D aerator device, is developed, and the lateral cavity and fin performance of the BLD-3-D aerator device are experimentally investigated. The findings show that, the relative lateral cavity length with backward lateral deflectors is shorter than that with current lateral deflectors under the same approach flow conditions, and on the basis of the results of the relative cavity length ratio between the lateral and bottom aerators the BLD-3-D aerator device is of remarkable performance for the water fin control thanks to the decrease of the relative lateral cavity length.

Relative-cavity-length-controlled slow light in a cascaded magnon-photon system

We theoretically discuss the input–output characteristics of the cascaded magnon-photon system by focusing on the slow light effect caused by positive group delay. When two identical magnon-photon systems are cascaded, compared with the single cavity system, the bandwidth of the whole system is wider, but the group delay is smaller. The change of the group delay is different from that of the general linear system. Moreover, by changing the cavity length of the two microwave cavities, the system is slightly mismatched and the group delay could obviously increase. The slow light effect could be controlled by relative cavity length. The slow light effect of the mismatched double cavity system is more sensitive to external magnetic field than that of the single cavity system. The results provide a way to manipulate the transmission rate in optical information networks and a new view about the interconnection of microwave cavities.

Numerical modelling of shock wave-boundary layer interaction control by passive wall ventilation

The normal shock wave-boundary layer interaction (SBLI) phenomenon is known to constitute a main factor limiting the aerodynamic performance in many aeronautical applications (transonic wings, helicopter rotor blades, compressor and turbine cascades). The interaction process highly disturbs the boundary layer, often causing flow separation and onset of large scale unsteadiness (e.g. airfoil buffet or supersonic inlet buzz). In certain conditions it may also initiate a dramatic increase of acoustic emission levels (e.g. high-speed impulsive noise). To limit the negative impact of the phenomenon various flow control strategies are implemented, here in a form of a passive control system realised by placing a shallow cavity covered by a perforated plate just beneath the shock. Details of the flow structure obtained by this method are studied numerically. Three distinctive experimental set-ups are considered with the interaction taking place: on a flat wall (transonic nozzle, ONERA), on a convex wall (curved duct, University of Karlsruhe), and on an airfoil (NACA 0012, NASA Langley). Depending on the relative cavity length the ventilation process leads to a transformation of the normal shock topology into: a large λ-foot structure (classical, short cavity), a system of oblique waves (extended cavity), or a gradual compression (full-chord perforation). The reference and flow control cases are simulated with the SPARC code (RANS) with Spalart–Allmaras turbulence and Bohning–Doerffer transpiration models. The results are compared with the measurements, emphasizing the streamwise evolution of the boundary layer profiles and integral parameters during the interaction. The prediction capabilities of the solver in terms of the shock wave-boundary layer interaction control by wall ventilation are assessed and presented in details for the investigated range of flow configurations and conditions.

Investigating the Effect of Aeration on the Flow Characteristics Around Under Pressure Tunnel Aerator Using Openfoam Open Source Software

Abstract The flow around the ramp embedded in a pressurized tunnel is divided into the various zones immediately downstream of the ramp, including the cavity and the main zone of flow above the shear layer. The aeration coefficient of the flow from the lower surface (inside the cavity)(βlower) is a function of non-dimensional numbers which aerator geometry parameters such as cavity length to ramp height Lc/tr is considered as one of the most important parameters. Therefore, in the present study, OpenFOAM software and RNG k-ε turbulence model were used to simulate the flow to study the aeration effect on flow characteristics, so the range of aeration coefficient as 0%<β<10% for four ramps with different tr/d ratios is Applied, then by increasing the aeration coefficient in the range of 0%<β<16%, The dependency range of relative Cavity length Lc/tr to (β) was evaluated. In order to verify performance of the numerical model, experimental results of Manafpour test were used. The results of the research indicate that in the aeration of flow with aerator ramp, the height of ramp has more influence relative to ramp angle. Moreover the main Effectiveness of the aeration coefficient on the cavity length was limited to air percent less than 10%.

Investigating the Effect of Ramp Geometry on the Flow Characteristics Around Under Pressure Tunnel Aerator Using OpenFoam Open Source Software

The flow around the ramp embedded in a pressurized tunnel is divided into various zones downstream of the ramp, including the cavity and the main zone of flow above the shear layer. Ramp angle and height are parameters that affect the flow characteristics such as cavity length, velocity, and pressure coefficient immediately downstream of the ramp. In this study, OpenFOAM open source software and RNG K-ε turbulence model were used to simulate the flow around the under pressure tunnel ramp. In order to investigate the effect of the ramp geometry on the flow in various relative air discharges 0<β<10, the range of height and the angle of the ramp as 5<θ<20 and 0.1<tr/d<0.4 were developed and simulated. The correlation coefficient between the numerical and experimental results for the relative cavity length is in the range of 0.9377≤R2≤0.9722 that indicates proper agreement between results. The result of the research shows that in both cases of fixed height of ramp and increasing ramp angle, and fixed angle of the ramp and increasing ramp height, the values of the cavity length and maximum turbulence intensity increase, and the minimum pressure values at the cavity zone bed are decreased. But in both cases, the sensitivity of the three mentioned parameters is higher than the ramp height increment.

Vibrational convection of heat-generating fluid in a rotating horizontal cylinder. The role of relative cavity length

Thermal convection of fluid with internal heat sources in a rotating horizontal cylinder with isothermal boundary and adiabatic ends is investigated experimentally. Under the action of gravity nonisothermal liquid oscillates in the cavity frame. These tidal oscillations generate the average mass force and, as a result, excite convection. The steady convection developing by this mechanism is called thermal vibrational. Centrifugal force in the considered case plays a stabilizing role. The objects of studying are the excitation thresholds of the averaged convection, heat transfer and the structure of convective flows. The parameters varying in the experiments are heat release rate, relative length of the cylinder and rotation velocity. It is found that the inertial waves which are generated near the ends of the cavity by tidal oscillations of nonisothermal liquid effect the convection. The intensity of flows excited by these waves is relatively low, but significant especially below the threshold of thermal vibrational convection. It is shown that the influence of inertial waves on heat transfer and structure of convective flows strongly depends on the cavity aspect ratio.

Research on cavitation characteristic of inducer

The inducer has significant effect on improving the cavitation characteristic of a centrifugal pump. The fact which can not be neglected is that the inducer itself is a kind of axial pump. Research on inducer's cavitation characteristic is very important. Several inducers were designed and modeled by Pro/E software. The mesh of flow field was done by ICEM and imported to ANSYS CFX to analyze the inducer's cavitation characteristic. The relationship between cavity length and head breakdown was discussed. With the decrease of NPSH, there is a slight increase in the head just prior to the decrease associated with head breakdown. This conclusion coincides with experimental results. The influence of backflow eddy on the inducer's cavitation characteristic was analyzed, and the change of backflow eddy in the process of cavitation was illustrated. It can be concluded that the correlation between the inducer head breakdown and the relative cavity length is very close which agrees well with the theoretical and experimental results. As the inlet pressure is decreased, inception almost always occurs in the tip vortex generated by the corner where the leading edge meets the tip. And backflow vortex gradually disappears in the process of cavitation.

Cavitation performance evaluation for a condensate pump

Cavitation in a condensate pump with specific speed of 95 m·m3s−1·min−1 was treated in this study. Cavitation performance for the pump was tested experimentally, and the steady state cavitating flows in the pump impeller were simulated by RANS method as well as a homogeneous cavitation model. It is noted that cavitating flow simulation reasonably depicted cavitation development in the pump. Compared to the tested results, the numerical simulation basically predicted later performance drops due to cavitation. Unfortunately, the cavitation simulation at the operation condition of 50% best efficiency point could not predict the head drop up to 3%. By applying the concept of relative cavity length cavitation performance evaluation is achieved. For better application, future study is necessary to establish the relation between relative cavity length and performance drop.

Thermal Transport in a Microchannel Exhibiting Ultrahydrophobic Microribs Maintained at Constant Temperature

This paper presents numerical results exploring the periodically repeating laminar flow thermal transport in a parallel-plate microchannel with ultrahydrophobic walls maintained at constant temperature. The walls considered here exhibit alternating microribs and cavities positioned perpendicular to the flow direction. Results describing the thermally periodically repeating dynamics far from the inlet of the channel have been obtained over a range of laminar flow Reynolds numbers and relative microrib/cavity module lengths and depths in the laminar flow regime. Previously, it has been shown that significant reductions in the overall frictional pressure drop can be achieved relative to the classical smooth channel laminar flow. The present predictions reveal that the overall thermal transport is also reduced as the relative size of the cavity region is increased. The overall Nusselt number behavior is presented and discussed in conjunction with the frictional pressure drop behavior for the parameter range explored. The following conclusions can be made regarding thermal transport for a constant temperature channel exhibiting ultrahydrophobic surfaces: (1) Increases in the relative cavity length yield decreases in the Nusselt number, (2) increasing the relative rib/cavity module length yields a decrease in the Nusselt number, and (3) decreases in the Reynolds number result in smaller values of the Nusselt number.

All-optical phase locking of two femtosecond Ti:sapphire lasers: a passive coupling mechanism beyond the slowly varying amplitude approximation.

Two independently tunable femtosecond Ti:sapphire lasers are passively synchronized with a stable relative carrier-envelope offset phase. By heterodyning the spectral overlap of the two frequency combs, we observe multiple regimes for the cavity length difference in which the relative round-trip phase slip is effectively locked to zero. The strong correlation of the femtosecond pulse trains is maintained over minutes without any external stabilization, and relative cavity length variations of 50 nm are compensated. The phase synchronization relies on phase-dependent cross-phase modulation, taking full advantage of the nonresonant optical nonlinearity of the shared gain medium, which is much faster than the optical cycle.

Investigation of the flow in an annular cavity in a cylindrical body in a supersonic stream

The results of an experimental investigation of the flow over an annular cavity in a cylindrical body are presented; the cavity-to-body diameter ratio was 0.7 and the incident flow Mach number was 2.84. Using the data on the pressure distribution and optical measurements of the flow pattern, the structure of the flow inside the cavity was studied on the relative cavity length range from 0.5 to 14 including the regimes with both open and closed separation zones.

Sticky pulses: two-color cross-mode-locked femtosecond operation of a single Ti:sapphire laser

We demonstrate mode-locked two-wavelength synchronized operation of a single cw-pumped Ti:sapphire laser. There is no active feedback or external cavity. The measured cross correlation is 105 fs. Cross mode locking is maintained for relative cavity length detunings as large as ±3 μm and wavelength offsets as large as 20 nm.

Gas-dynamic heating in cavities under the influence of pressure pulsations at the entrance

An investigation has been made of the gas-dynamic heating of gas in nearly closed cavities (tubes, channels, etc.) under the influence of given pressure pulsations (with and without a discrete component) at the entrance. The results are given of measurements of the wall temperature of the cavities and also the power of the gas-dynamic heating as a function of the relative cavity length 10< ln/dn < 300, the relative level of the pressure pulsations σ0/p < 0.5 at the entrance, and the magnitude of the static pressure in the range p = 2–10 kg/cm2. It was established that with increasing p and especially σ0/p the power of the gas-dynamic heating increases strongly.