Heat Transfer Surface Geometries Research Articles

Multi-objective optimization and system evaluation of recuperated helicopter turboshaft engines

Targeting higher efficiency and lower emissions, the employment of recuperators in helicopters is well recognized as an attractive technical strategy to enhance the operational capabilities. Primary surface recuperator (PSR) is proposed for such applications, as its favorable characteristics of good heat transfer performance and compact structure enable a light weight recuperator design in the aeroengine industry. Aimed at designing a PSR potentially applicable to rotorcraft powerplants, initially, promising heat transfer surface geometries are evaluated based on their aerothermal performance. Subsequently, through the execution of multi-objective genetic algorithm optimization, interdependencies between recuperator weight and thermal effectiveness are quantified under specific constraints, with respect to the selected heat transfer surface geometries. Eventually, acquired results of optimal designs are further analyzed within a multidisciplinary simulation framework for performance assessment of complete helicopter operations under given flight conditions. It is found from helicopter mission analysis that the optimum trade-off between fuel saving benefits and associated recuperator weight penalty is attained for the employed effectiveness of 76.1% with recuperator weight of 27.48 kg. The proposed methodology could be adopted as a cost-effective and computationally-efficient tool for the multidisciplinary design and optimization of rotorcraft powerplant systems incorporating high performance recuperators.

Heat transfer and hydraulic resistance in channels with spherical protrusions

This article presents an analytical overview of experimental studies of hydraulic resistance and heat transfer in channels with disturbed flow surfaces in the form of systems of spherical protrusions in a wide range of flow and design parameters. A uniform approach to generalization of experimental data by various authors has been developed considering differences in the geometry of heat-transfer surfaces. Uniform universal generalizing dependences have been obtained for the hydraulic resistance and heat transfer coefficients in channels with spherical protrusions.

Generalization of Experimental Data on Heat Transfer and Hydraulic Resistance of Channels with Spherical Protrusions

Experimental investigations of hydraulic resistance and heat transfer in channels with intensifiers in the form of systems of spherical protrusions have been generalized in a wide range of working and design parameters. A unified approach to the correlation of experimental data of various investigators with account for the differences in the geometry of heat transfer surfaces has been worked out. Unified universal dependences for the coefficients of hydraulic resistance and heat transfer in channels with spherical protrusions are obtained.

An engineering procedure for air side performance evaluation of flat tube heat exchangers with louvered fins

An accurate evaluation of possible air side heat transfer surface geometries is a prerequisite for optimal heat exchanger design. Aiming for practical engineering applicability a simplified and transparent analytical procedure for the assessment of louvered fin and flat tube heat exchanger geometries and the calculation of fin parameters that enable maximal performance for given boundary conditions has been developed. The proposed method comprises determining fins temperature profiles and effective heat transfer temperature difference, introduction of a relative heat transfer surface area, as well as the utilization of recent experimentally obtained heat transfer correlations confirmed for the observed range of boundary conditions. The proposed methodology is validated through comparison with experimental and numerical results of other authors.

Confined jet impingement of liquid nitrogen onto different heat transfer surfaces

Jet impingement of liquid nitrogen owns many applications in the cryogenic cooling aspects, such as, cooling of high-power chips in the electronic devices and cryoprobes in the cryosurgery. In the present study, we systematically investigated the confined jet impingement of liquid nitrogen from a tube of about 2.0 mm in diameter onto the heat transfer surfaces of about 5.0 mm in basement diameter with different heat transfer surface geometries and conditions, i.e., flat surface, hemispherical surface and flat surface with a needle. The effects of many influential factors, such as, the geometry of the heat transfer surface, jet velocity, distance between the nozzle exit and heat transfer surface, heat transfer surface condition, and some other, on the heat transfer were investigated. The heat transfer correlations were also proposed by using the experimental data, and it was found that the heat transfer mechanism of liquid impingement in the confined space was dominated by the convective evaporation rather than the nucleate boiling in the present case. The critical heat flux (CHF) of the confined jet impingement was measured and the visualization of the corresponding flow patterns of the confined jet impingement of liquid nitrogen was also conducted simultaneously to understand the heat transfer phenomena.

Sizing problem for a cross flow heat exchanger with wing-type vortex generator

In the present study, sizing of a single pass cross flow heat exchanger with unmixed fluid streams has been investigated. The heat exchanger is a cross flow heat exchanger. It has overall dimensions of 20 × 20 × 20 cm. Two the most common heat exchanger design problems are the rating and sizing problem. Sizing problems deal with designing an exchanger and determining its physical size to meet the specified heat duty, pressure drops and other considerations. It means the determination of the exchanger construction type, flow arrangement, heat transfer surface geometries and materials, and the physical sizes of an exchanger to meet specified heat transfer and pressure drop. In this study, the physical size (length, width, height, mass flow rates of both fluids and surface areas on each side of the exchanger) are determined. Inputs to the sizing problem are surface geometries, fluid mass flow rates, inlet and outlet fluid temperatures and pressure drop on each side. Dimensions of L a , L b , and L c for the selected surfaces were investigated such that the design meets the heat duty and pressure drops on both sides exactly.

Recuperator considerations for future higher efficiency microturbines

First-generation microturbines are based on the use of existing materials and proven technology, and with low levels of compressor pressure ratio and modest turbine inlet temperatures, have thermal efficiencies approaching 30% for turbogenerators rated up to 100 kW. For such small machines the goal of advancing beyond this level of performance is unlikely to include more complex thermodynamic cycles, but rather will be realised with higher turbine inlet temperatures. Advancing engine performance in this manner has a significant impact on recuperator technology and cost. In the compact heat exchanger field very efficient heat transfer surface geometries have been developed over the last few decades but further improvements perhaps using CFD methods will likely be only incremental. Automated fabrication processes for the manufacture of microturbine recuperators are in place, and on-going developments to facilitate efficient higher temperature operation are primarily focused in the materials area. Based on the assumptions made in this paper it is postulated that in the 100 kW size the maximum thermal efficiency attainable for an all-metallic engine is 35%. To achieve this the recuperator cannot be designed in an isolated manner, and must be addressed in an integrated approach as part of the overall power conversion system. In this regard, temperature limitations as they impact the recuperator and turbine are put into perspective. In this paper there is strong focus on recuperator material selection and cost, including a proposed bi-metallic approach to establish a cost-effective counterflow primary surface recuperator for higher temperature service. If indeed there is a long-term goal to achieve an efficiency of 40% for small microturbines, it can only be projected based on the utilisation of ceramic hot end components. Alas, the high temperature component that has had the minimum development in recent years to realise this goal is the ceramic recuperator, and efforts to remedy this situation need to be undertaken in the near future.

Analysis of Heat Transfer Surface Geometries for Dry Cooling Tower Applications

An analysis of heat exchanger surface geometries for the purpose of reducing dry cooling tower cost is presented. Two sets of results are derived. The first set can be used to evaluate heat transfer surface geometries in an attempt to select those most suitable for dry cooling tower applications. The second set of results can be used to direct research and development efforts toward developing better geometries for dry cooling tower applications. The first set of results is general and is applicable to all heat exchanger surface geometries. The second set is valid only for helical round or continuous fins having smooth, serrated, or cut fins and for staggered and in-line tube arrangements. The methods developed in this paper are not restricted to dry cooling towers per se, but are valid for other applications of fin tube heat exchangers as well.

Heat-Transfer and Flow-Friction Characteristics of Crossed-Rod Matrices

The paper presents a continuation of the program on porous media heat-transfer and flow-friction behavior previously covered in References [2b] and [3b]. All the previous results of interest to the designer on woven-screen matrices and crossed-rod matrices of a random configuration are summarized here. In addition, new design results for the regular in-line and regular staggered crossed-rod-matrix configurations are reported. Matrices of the type considered here may find application as heat-transfer surface geometries for nuclear-reactor fuel elements, for electrical resistance heaters and for periodic-flow-type heat exchangers used for gas-turbine regenerators, and some air-conditioning applications.