Experimental and numerical investigation of heat transfer characteristics in a square channel with various truncated ribs

Impact cooling is an effective way to enhance heat transfer, especially in the gas turbine blades. In the leading edge of the blade where has the high heat load, jet impingement cooling is widely used due to its high heat transfer characteristic in stagnation region. The focus is on finding a cooling structure that can improve the heat transfer effect of the internal impact structure at the leading edge without increasing the internal flow resistance. In this paper, using transient liquid crystal experiments researches for the flow and heat transfer characteristics of a semi-circular structure, which is simplified from the real blade leading edge ’s inside surface and have different rib structures. This paper studies five cases:no rib, round-shaped raised structure, oblique rib, round-shaped raised structure and oblique rib and span-wise rib and arc rib to find their heat transfer and flow characteristics. Some rib-shaped protrusion has three heights, which are 30%, 50%, 70% of the impact distance H. Experimental conditions of Reynolds number are Re = 10000, 15000, 20000, 25000, 30000. The experimental verification results show that the internally strengthened heat transfer structures studied in this paper can improve the heat transfer effect of the leading edge array of the turbine blade impact target surface without increasing the flow resistance. The structure with both oblique ribs and round-shaped raised structures has the highest surface average Nusselt number of the target plate and the lowest discharge coefficient of the channel. The structure with both span-wise ribs and arc ribs has a staggered high heat transfer area distribution, which can maybe use in some special cases.

Influence of perforated triple twisted tape on thermal performance characteristics of a tube heat exchanger

Heat transfer and flow characteristics in a rectangular channel with combined delta winglet inserts

In order to increase entry gas temperature and improve the efficiency of gas turbine, steam is used as a coolant instead of air. Much research has been carried out on the closed circuit steam cooling of vanes substituted with film-cooling using compressor air in recent years. Furthermore, by studying the steam flow and heat transfer characteristics in rib ducts, this investigation focuses on establishing the basis of steam cooling technology application in complex flow field of internally-cooled turbine vane. In this paper, a report and assessment of RSM method based on SSG turbulence model is performed with commercial computational fluid dynamics software ANSYS CFX. The numerical results of heat transfer coefficient and friction factors in square channels with 90 degree rib turbulators for Reynolds numbers of 10 000, 30 000 and 60 000 are compared with the experimental data from Han’s. It is found that the obtained heat transfer coefficient distributions and friction factors match well with SSG turbulence model. In addition, the heat transfer distribution and pressure drop of steam-cooled ducts are predicted under the same work conditions by using dry real gas model. The Reynolds number could be correlated with the Nusselt number. The impact of steam physical properties on heat transfer performance are researched detailedly by respectively changing the steam superheat and entry pressure. The results indicate that the RSM method with a suitable turbulence model is valuable for the air-cooled and steam-cooled duct with the acceptable engineering accuracy (less than 20%). Comparing the cooling efficiency between steam and air under the same operation condition, the advantage of using cooling steam is evident than using cooling air. Furthermore, the efficiency of the whole gas turbine system will be greatly improved through using the closed loop steam cooling system. Changing the steam superheat and entry pressure, it has little effect on the steam flow and heat transfer characteristics. Increasing the steam overheat would raise the friction factor. Contrarily, enhancing the entry pressure would decrease the friction factor.

In this paper, 3D numerical simulations are conducted in heat transfer enhanced tubes with internally roughened dimples in the range of Re = 2000 to 11,000. The fluid flow and heat transfer characteristics are fully understood by the local and overall friction factors, Nusselt number, and Colburn j factor. The local mean friction factor has a periodic change due to the periodic dimple distribution, the eddy zone and high-pressure region of every dimple greatly influence the working fluid for heat transfer enhancement. In this numerical simulation, five samples are employed to study the detailed heat transfer characteristics. The results show that the dimpled tube is a new kind of heat transfer enhanced tube with the excellent heat transfer performance and low resistance. It is found that the effects of dimples on the heat transfer performance can be well described by the field synergy principle. The effect of different dimple arrangements is very little, which is always within 2%. But the effect of dimple size on heat transfer and fluid flow performance is very significant, thus there exists a tube with an optimum dimple size among the tubes investigated in this paper. By integrated performance evaluation of NUF0/NU0F, a maximum of about 60% heat transfer enhancement with the same friction penalty can be gained by the optimal dimpled tube.

In this study, the effects of fin tip thickness and fin root thickness of integral rolled spiral finned tube bundles on flow resistance, heat transfer performance and heat transfer and flow exergy destruction were investigated via mathematical simulation. The correlations between heat transfer and flow resistance performance were fitted with dimensionless numbers. The optimized parameters with performance evaluation criteria (PEC) as the objective were obtained using methods involving computational fluid dynamics and machine learning. The results show the effects of fin tip thickness and fin root thickness on the Nusselt number (Nu), Euler number (Eu), PEC, heat transfer exergy destruction (ExT) and flow exergy destruction (ExP) as obtained via mathematical simulation. A new mathematical correlation is proposed for predicting the Nu and Eu of integral rolled spiral finned tube bundles. Among the four optimization models tested, the random forest regression algorithm was the most accurate algorithm for PEC prediction models. In the studied range, the optimal parameters were a fin tip thickness of 2 mm and a fin root thickness of 3.5 mm. Compared with the initial parameters, when the Reynolds number was 20,380, the PEC increased by 2.53%, the ExP increased by 2.37% and the ExT decreased by 7.96%.

Numerical study on the heat transfer performance and efficiency in a rectangular duct with new winglet shapes in turbulent flow

Heat transfer and pressure drop characteristics of heat exchangers based on triply periodic minimal and periodic nodal surfaces

3D numerical simulation on the shell side heat transfer and pressure drop performances of twisted oval tube heat exchanger

An experimental investigation was accomplished to evaluate the performance of heat transfer for turbulent flow through a tube with helical tape inserts. The mild steel helical tape inserts with different twist ratios of 1.88, 3.13, 4.69, 6.41 and 7.81 were used in the flow field. Heat transfer and pressure drop data were prompted for a wide range of Reynolds number from 7200 to 50,000. The experimental results indicated that the Nusselt number, friction factor and thermal performance factor were increased with decreasing twist ratio. The results also showed that helical tape inserts of different geometries in a circular tube enhanced the heat transfer rate significantly with corresponding increase in friction factor. Nusselt number and friction factor for the tube with inserts were found to be increased up to 260 and 285 %, respectively, than those over the plain tube values at the comparable Reynolds number. The heat transfer performance was evaluated and found to be 44 % higher compared to the plain tube based on the constant blower power. Finally, new correlations were proposed for the twist ratios ranging from 1.88 to 7.81 for predicting the heat transfer, friction factor and thermal performance factor for turbulent flow through a circular tube fitted with helical tape inserts.

Experimental investigation on convective heat transfer and rheological characteristics of Cu–TiO2 hybrid nanofluids

The heat exchanger is widely applied to many axial piston machines, and its structure significantly affects the flow and heat transfer characteristics. The fluid flow and heat transfer characteristics of a dimple-type heat exchanger in the axial piston pump are numerically investigated. The variations of the Nusselt number, friction factor and heat transfer performance with the Reynolds number, and the dimple radius to depth are obtained. The results show that both the Nusselt number and the resistance coefficient of the spherical dimple channel are higher than those of the triangle dimple one. Thus, from the view of the heat transfer performance, the spherical dimple channel is better. Furthermore, the friction factor increases as the dimple radius to depth increases. With the increase in the dimple radius to depth ratio, the shear stress at the wall of the spherical dimpled channel gradually decreases and then reduces the wall friction resistance. Comparing with the triangle dimple channel, the friction factor in the spherical dimple channel is 0.044–0.022, which reduces about 9%. When the radius–depth ratio is set to 0.1, the effectiveness factor for the spherical dimpled channel has better performance.

Effect of splitter angles and orientations attached to pin fin on heat transfer and hydraulic characteristics in a jet impingement rectangular channel

The effects of concentration of water‐based MgO nanofluid on heat transfer and pressure drop characteristics of a shell‐and‐tube heat exchanger is modelled and numerically simulated by using five kinds of volume concentrations of 0%, 0.2%, 0.35%, 0.4%, and 0.5%. Validations for modelling and algorithm are performed by experiments with respect to the parameters of average heat transfer and overall heat transfer coefficient by means of distilled water‐based medium, which are in well line with measurement data. The overall heat transfer coefficient, Nusselt number, pressure drop, friction factor, Prandtl number and performance evaluation criteria are predicted using commercial software Fluent 2020R2. Results showed that an increase in volume concentration of nanofluid enhance the heat transfer performances. Compared to water‐based medium, the peak values of the overall heat transfer coefficient and the Nusselt number are 14.52% and 13.02%, referring to the threshold of volume concentration of 0.35%.

Effect of vacuum environment on air side heat and mass transfer characters of plain fin-tube heat exchangers