Jan Wajs

Experimental investigation of M-shape heat transfer coefficient distribution of R123 flow boiling in small-diameter tubes

This article presents a study of flow boiling of R123 in two small-diameter silver tubes with inner diameters of 1.15 mm and 2.3 mm. The experiments have been accomplished for a wide range of quality variation (0.01–0.9), mass flow rate (650–3000 kg/m2s), and heat fluxes (40–80 kW/m2). The saturation temperature ranged from 30 to 70°C. In the experiments a peculiar distribution of heat transfer coefficient leading to development of two maxima in its distribution with respect to quality has been observed. Such behavior was seen in both sizes of tubes.

Effect of Surface Roughness on Thermal-Hydraulic Characteristics of Plate Heat Exchanger

In the paper the experimental analysis of passive heat transfer intensification in the case of modeled plate heat exchanger is conducted. The plate heat exchanger is chosen for the analysis because this kind of heat exchangers could be prospectively applied in the ORC systems, however other areas or application are equally possible. The experimental set-up was assembled at the Department of Energy and Industrial Apparatus of Gdansk University of Technology. The passive intensification was obtained by a modification of the heat transfer surface. The roughness of surface was increased by use of glass shot.During the experiment single-phase convective heat transfer in the single phase system was studied. The experiment was done in two stages. In the first stage the model of commercial plate heat exchanger was investigated, while in the second stage the identical one but with modified heat transfer surface. Model of heat exchanger consisted of three plates. The direct comparison of thermal and flow characteristics between both devices was possible due to assurance of equivalent conditions at the inlet to the system.The thermal and hydraulic characteristics are presented. The thermal analysis shows that in some range of heat flux density the overall heat transfer coefficient was higher for the commercial heat exchanger, while for the other was higher for the heat exchanger with modified surface. The influence of larger roughness on heat transfer cannot unequivocally be evaluated. Therefore as the next step the systematic investigations of model heat exchangers (only with one hot and one cold passage) will be conducted.

Possibilities of heat transfer augmentation in heat exchangers with minichannels for marine applications

Abstract In the paper, new trends in the development of microchannel heat exchangers are presented. The exchangers developed in this way can be applied in marine industry. Main attention is focused on heat exchanger design with reduced size of passages, namely based on microchannels. In authors′ opinion, future development of high power heat exchangers will be based on networks of micro heat exchangers.

Thermal performance of a prototype plate heat exchanger with minichannels under boiling conditions

To solve the problem and to meet the requirements of customers in the field of high heat fluxes transfer in compact units, a new design of plate heat exchanger with minichannels (minichannels PHE) was proposed. The aim was to construct a compact heat exchanger of high effectiveness for the purpose of household cogeneration ORC system. In this paper the experimental analysis of an assembled prototype of such compact heat exchanger was described. The attention was paid to its thermal performance and the heat transfer coefficients under the boiling conditions. Water and ethanol were chosen as working fluids. The maximal value of transferred heat flux was about 84 kW/m2, while of the overall heat transfer coefficient was about 4000 W/(m2K). Estimated values of heat transfer coefficient on the ethanol (boiling) side reached the level of 7500 W/(m2K). The results are promising in the light of future applications, for example in cogeneration ORC systems, however further systematic investigations are necessary.

Local Nusselt number evaluation in the case of jet impingement

Jet impingement still is one of demanding cases regarding computational fluid dynamics, due to its highly turbulent behaviour, with occurrence of turbulent-laminar transition. Even recently developed methods exhibit some drawbacks - RANS based simulations lack accuracy, LES and DNS based ones require too much computational time. Hybrid methods also exist, but their development and validation is in progress. Nevertheless, CFD application can play major role in the investigation of jet impingement phenomena. While the flat surface impingement is widely discussed in the literature, there is lack of data regarding non-flat surfaces - the ones that might exist for example in the heat exchangers. In the following paper, the numerical simulation of both flat and non-flat surfaces single jet impingement is presented, with the aim of precise description of the turbulence models impact on the thermal and hydrodynamic results. Choice of turbulence model is crucial for sufficient calculation outcome. Only the complex analyses, shown in the article, including the turbulence and momentum budgets comparison between particular models, can reveal significant and meaningful differences.

High Performance Tubular Heat Exchanger with Minijet Heat Transfer Enhancement

ABSTRACT In the paper, the original cylindrical heat exchanger with minijets (MJHE) was introduced. The systematic experimental analysis of the prototype heat exchanger was described with special attention paid to such parameters as the heat transfer effectiveness, heat transfer rates, overall heat transfer coefficients, and pressure drop. The heat transfer coefficients were determined based on Wilson plot method, the most suitable approach for heat transfer coefficient determination in exchangers of complex geometry. The thermal-hydraulic characteristics of the in-house manufactured prototype of MJHE in water–water and gas–water configuration are also presented. The experimental results were compared with the predictions from well-known correlations found in the published research papers dedicated to the free-surface and submerged types of jets.

Alternative cogeneration thermodynamic cycles for domestic ORC

The Organic Flash Cycle (OFC) is suggested as a vapor power cycle that could potentially improve the efficiency of utilization of the heat source. Low and medium temperature finite thermal sources are considered in the cycle. Additionally the OFC’s aim is to reduce temperature difference during heat addition. The study examines 2 different fluids. Comparisons are drawn between the OFC and an optimized basic Organic Rankine Cycle (ORC). Preliminary results show that ethanol and water are better suited for the ORC and OFC due to higher power output. Results also show that the single flash OFC achieves better efficiencies than the optimized basic ORC. Although the OFC improves the heat addition exergetic efficiency, this advantage was negated by irreversibility introduced during flash evaporation.

Management of Low-Temperature Heat Source by ORC Aided by Additional Heat Source

In the paper presented is a concept to utilize waste heat from the power plant with the aid of the low-temperature ORC cycle. The ORC system is heated from two heat sources, the first one being the flow rate of waste heat obtained from the exhaust gases. Subsequently, the working fluid in the cycle is additionally heated by the condensing steam from the low pressure turbine extraction points increasing in such way the level of temperature of working fluid before turbine to 120°C. Examination of the results enables to conclude that the overall efficiency of the cycle increased from =51.958% to =52.304%. That is due to the fact that additional heat enabled to evaporate more working fluid. The total generated power increased to the value of NelRU=915.85MWe, which corresponds to about 1.5% increase in power.

Organic Rankine Cycle as Bottoming Cycle to a Combined Brayton and Clausius - Rankine Cycle

A preliminary evaluation has been made of a possibility of bottoming of a conventional Brayton cycle cooperating with the CHP power plant with the organic Rankine cycle installation. Such solution contributes to the possibility of annual operation of that power plant, except of operation only in periods when there is a demand for the heat. Additional benefit would be the fact that an optimized backpressure steam cycle has the advantage of a smaller pressure ratio and therefore a less complex turbine design with smaller final diameter. In addition, a lower superheating temperature is required compared to a condensing steam cycle with the same evaporation pressure. Bottoming ORCs have previously been considered by Chacartegui et al. for combined cycle power plants [ Their main conclusion was that challenges are for the development of this technology in medium and large scale power generation are the development of reliable axial vapour turbines for organic fluids. Another study was made by Angelino et al. to improve the performance of steam power stations [. This paper presents an enhanced approach, as it will be considered here that the ORC installation could be extra-heated with the bleed steam, a concept presented by the authors in [. In such way the efficiency of the bottoming cycle can be increased and an amount of electricity generated increases. A thermodynamic analysis and a comparative study of the cycle efficiency for a simplified steam cycle cooperating with ORC cycle will be presented. The most commonly used organic fluids will be considered, namely R245fa, R134a, toluene, and 2 silicone oils (MM and MDM). Working fluid selection and its application area is being discussed based on fluid properties. The thermal efficiency is mainly determined by the temperature level of the heat source and the condenser conditions. The influence of several process parameters such as turbine inlet and condenser temperature, turbine isentropic efficiency, vapour quality and pressure, use of a regenerator (ORC) will be presented. Finally, some general and economic considerations related to the choice between a steam cycle and ORC are discussed.