Heat Transfer Characteristics Of Film Research Articles

Numerical investigation on conjugate heat transfer characteristics of film and vortex composite cooling under rotating conditions

This paper numerically investigates the vortex and film composite cooling performance under rotating conditions. The cooling performance of the adiabatic and conjugate models is compared under the range of 0–4000 r/min. The conjugate model contains the fluid region: the cascade, the film holes, and the internal vortex cooling chamber, as well as the solid region: the blade material between the internal flow and the mainstream flow. The adiabatic model is established by removing the blade material part in the conjugate model. The dimensionless temperature θ inversely proportional to the temperature is adopted. Results show that the blade leading edge temperature doesn’t vary linearly with the rotating speed. The stagnation line of the mainstream flow on the blade leading edge moves from the pressure surface to the suction surface. The maximum θ appears at 1500 r/min when the stagnation line stays certainly on the row of film holes located on the pressure surface and is 7.86% higher than the minimum θ. The minimum θ appears at 2500 r/min when the stagnation line stays on the position between the rows of film holes. The distribution of θ is much uniform, and the value of θ is much higher in the conjugate cases than the adiabatic cases due to the heat conduction through the blade material. The highest aerodynamic parameter appears at 2000 r/min due to its relatively low blade leading edge temperature and low coolant consumption and is 41% higher than the aerodynamic parameter at 0 r/min.

Film Cooling Performance Enhancement by Upstream V-shaped Protrusion/Dimple Vortex Generator

Film cooling is an effective technique to isolate the heat flux from high-temperature mainstream, but the counter-rotating vortex pair (CRVP) generated by the interaction between film jet and mainstream leads to serious jet-off and poor film coverage. To enhance film cooling performance, a v-shaped protrusion/dimple vortex generator is arranged at the upstream location of film hole for suppressing the CRVP. The flow and heat transfer characteristics of film cooling with four kinds of vortex generators, v-shaped protrusion vortex generator (VSPVG), v-shaped dimple vortex generator (VSDVG), anti-v-shaped protrusion vortex generator (AVSPVG) and anti-v-shaped dimple vortex generator (AVSDVG), are investigated in detail over the blowing ratio ranging from 0.5 to 1.5 by solving three dimensional Reynolds-Averaged Navier-Stokes (RANS) equations with Realizable k-ε turbulence model. Results indicate that both the v-shaped vortex generators can generate additional anti-counter-rotating vortex pairs (ACRVP) to weaken the CRVP, while both the anti-v-shaped vortex generators will enlarge the CRVP. The arrangement of VSPVG or VSDVG is proved to enhance film cooling performance significantly, especially the VSPVG, which improves the adiabatic film cooling effectiveness by 82.3%, 187.9%, 157.7%, 162.2% and 145.2% respectively corresponding to the BR=0.5, 0.75, 1, 1.25 and 1.5 compared with Baseline case. Considering the total pressure loss, it is found that the enhancement of film cooling effectiveness comes at the cost of slightly increased total pressure loss. In further study, the effect of VSPVG height on film cooling performance is also discussed. It is observed that the adiabatic film cooling effectiveness increases with an increase of the height, and the total pressure loss also inescapably increases, but it is worth celebrating that compared with Baseline case, the higher film cooling effectiveness and lower total pressure loss coefficient is obtained when the height is lower than 0.15D.

Investigation on flow field and heat transfer characteristics of film cooling with different swirling directions for coolant flow

In this paper, a hexagonal prism inlet chamber is used to form a swirling flow for the film cooling, and three kinds of compound angle of film hole ( γ = 10°, 20°, 30°) with clockwise swirling or counterclockwise swirling are used for numerical simulation studies. The influence of different compound angles of film hole and the swirling directions for the film cooling effectiveness are obtained. The results show that the film cooling effectiveness and spanwise cooling coverage range of the clockwise swirling or counterclockwise swirling flow both are low when the compound angle of film hole is 10°. With the increasing compound angle of film hole, the kidney shaped vortex of film hole exit gradually weakens until it disappears, which reduces the entrainment effect by the coolant jet. So that the spanwise coverage range of two swirling modes is obviously improved. When the compound angle of film hole is 30° compared to 10°, the average spanwise film cooling effectiveness of clockwise swirling and counterclockwise swirling are increased by about 133.75 and 212.6%, respectively. The average spanwise film cooling effectiveness on the downstream of film hole for counterclockwise swirling is increased by about 140% compared with clockwise swirling.

Experimental investigation on heat transfer characteristics of film cooling using parallel-inlet holes

Experimental investigations on the film cooling characteristics of parallel-inlet film holes are carried out. There are four kinds of film hole arrangements: one-row, two-row(staggered arrangement), two-row(aligned arrangement) and three-row arrangement. Experimental results show that film hole arrangement and blowing ratios have great effects on film cooling effectiveness and heat transfer coefficient(HTC). Film cooling effectiveness is increased with the increase of blowing ratios, but the HTC is decreased with the increment of blowing ratio. The film cooling effectiveness of single-row film hole(d = 7 mm) is the best of all test pieces. As for staggered arrangement design, its HTC is larger than that of aligned arrangement at the same position. In addition, the HTC is increased with the decrement diameter of film hole for all kinds of film hole arrangement patterns. For film cooling, a smaller HTC means a better film cooling effectiveness.

A numerical study on flow and heat transfer characteristics of film cooling with a compound angle hole

A numerical simulation has been performed for the investigation of flow and heat transfer characteristics of a film cooling system injected through a hole with compound angle orientation. The finite volume method is employed to discretize the governing equations based on the non-orthogonal coordinate with non-staggered variable arrangement. In order to analyze flow and heat transfer characteristics, velocity, temperature, aerodynamic loss coefficient, skin friction and vorticity are calculated with the variation of the skew angle. The maximum longitudinal vorticity and aerodynamics loss depend strongly on the skew angle. For the symmetric case of β=0 deg, a pair of counter-rotating vortices are formed and the maximum value of the film cooling effectiveness has appeared in the center plane where the skin friction is the minimum. For the skew angle of β=30 deg and above, only one strong counter-clockwise vortex remains in the downstream region and the maximum value of the film cooling effectiveness are obtained on the right side of the vortex. The predicted results for the film cooling effectiveness show good agreements with previous experimental data except the near-hole region.

Multiple Jets in a Crossflow: Detailed Measurements and Numerical Simulations

The fluid mechanics and heat transfer characteristics of film cooling are three-dimensional and highly complex. To understand this problem better, an experimental study was conducted in a low-speed wind tunnel on a row of six rectangular jets injected at 90 deg to the crossflow (mainstream flow). The jet-to-crossflow velocity ratios (blowing ratios) examined were 0.5, 1.0, and 1.5, and the jet spacing-to-jet width ratio was 3.0. No significant temperature difference between jet and crossflow air was introduced. Mean velocities and six flow stresses were measured using a three-component laser-Doppler velocimeter operating in coincidence mode. Seeding of both jet and cross-stream air was achieved with a commercially available smoke generator. Flow statistics are reported in the form of vector plots, contours, and x-y graphs, showing velocity, turbulence intensity, and Reynolds stresses. To complement the detailed measurements, flow visualization was accomplished by transmitting the laser beam through a cylindrical lens, thereby generating a narrow, intense sheet of light. Jet air only was seeded with smoke, which was illuminated in the plane of the light sheet. Therefore, it was possible to record on video tape the trajectory and penetration of the jets in the crossflow. Selected still images from the recordings are presented. Numerical simulations of the observed flow field were made by using a multigrid, segmented, k–ε CFD code. Special near-wall treatment included a nonisotropic formulation for the effective viscosity, a low-Re model for k, and an algebraic model for the length scale. Comparisons between the measured and computed velocities show good agreement for the nonuniform mean flow at the jet exit plane. Velocities and stresses on the jet centerline downstream of the orifice are less well predicted, probably because of inadequate turbulence modeling, while values off the centerline match those of the experiments much more closely.

Heat transfer and the low limit of film boiling

This paper presents a state-of-the-art review of film boiling heat transfer. Particular emphasis is given to heat transfer characteristics of film boiling in various kinds of liquids including liquid metals and to the enhancement of the low limit, T min, of film boiling for improving and controlling the cooling process. Experimental data of both liquid metals and nonmetallic liquids are compared with theoretical relationships of film boiling heat transfer. Intermittent and local liquid-solid contacts, which are directly detected, cause an earlier onset of transition from film to transition boiling during the cooldown of high-temperature metals coated with a thin layer of low thermal conductivity material. A model to evaluate the enhancement of T min is also mentioned.

Film boiling characteristics of potassium droplets on heated plate.

For providing background information on the possible vapor explosion in the event of a core disruptive accident of LMFBRs, an experiment was conducted on the film boiling characteristics of liquid metal potassium in association with the Leidenfrost phenomenon. In a steel container filled with Ar gas, K droplets were put on a joule-heated plate of 316-SS or Ta. The behaviors of droplet were observed by a camera and a color VTR through viewports. The experimental conditions were the Ar pressure 1 bar, the initial K temperature 350~760°C, and the plate temperature 900~1,250°C for 316-SS and 1,100~1,600°C for Ta. Stable film boiling known as Leidenfrost phenomenon was observed for a high temperature condition of the plate, whereas an instantaneous break-up of droplet with extensive vaporization occurred for a low temperature. The heat transfer characteristics of film and transition boiling regions were obtained by estimating the heat flux from the volumetric reducing rate of droplet due to vaporization. The r...