M. De Paepe

Thermo-hydraulic design of earth-air heat exchangers

Earth-air heat exchangers, also called ground tube heat exchangers, are an interesting technique to reduce energy consumption in a building. They can cool or heat the ventilation air, using cold or heat accumulated in the soil. Several papers have been published in which a design method is described. Most of them are based on a discretisation of the one-dimensional heat transfer problem in the tube. Three-dimensional complex models, solving conduction and moisture transport in the soil are also found. These methods are of high complexity and often not ready for use by designers. In this paper, a one-dimensional analytical method is used to analyse the influence of the design parameters of the heat exchanger on the thermo-hydraulic performance. A relation is derived for the specific pressure drop, linking thermal effectiveness with pressure drop of the air inside the tube. The relation is used to formulate a design method which can be used to determine the characteristic dimensions of the earth-air heat exchanger in such a way that optimal thermal effectiveness is reached with acceptable pressure loss. The choice of the characteristic dimensions, becomes thus independent of the soil and climatological conditions. This allows designers to choose the earth-air heat exchanger configuration with the best performance.

An experimentally validated and parameterized periodic unit-cell reconstruction of open-cell foams

The physical behavior of open-cell foams depends on their microscopic structure. An open-cell geometrical model is proposed, which can serve as the basis for a future macroscopic analysis. The strut geometry is of particular interest, as it is reported to have substantial influence on the occurring thermo-hydraulic and mechanical phenomena. Axial strut size variation, as well as the porosity dependence of shape is quantified and included in a geometrical model. The foam is generated by placing the struts on an elongated tetrakaidecahedron. The required input parameters for the model are two cell dimensions, corresponding to the mean transverse and conjugate diameters of the ellipse encompassing a cell, and the strut cross-sectional surface area at its midpoint between two nodes. The foam geometry is generated iteratively, as porosity is used as validation. A high resolution micro-computed tomography scan is performed to measure the three parameters, the resulting porosity and surface-to-volume ratio. This allows to validate the model. The predictions are found to be within measurement accuracy. A numerical implementation of the model in the preprocessor of a commercial CFD package is demonstrated.

Technological and economical analysis of water recovery in steam injected gas turbines.

Steam injected gas turbines are an interesting technology for co-generation applications. In these gas turbines the heat of the exhaust gases is used to produce steam. This steam is injected in the combustion chamber, resulting in a high specific power and a high thermal efficiency. A major disadvantage of steam injected gas turbines is the large water consumption. Placing a condenser in the cycle makes it possible to recover all the injected steam. An analysis is made of different types of condensers. Condensers based on finned tubes and direct-contact-condensers are considered. The dimensions of the condensers are determined for existing steam injected gas turbines. Furthermore, the investment costs and the exploitation costs for each type are compared.

Cycle improvements to steam injected gas turbines.

The efficiency and specific power of the steam injected gas turbine is analysed by modelling the thermodynamic cycle. In this model special attention is paid to the blade cooling. The basic cycle as well as cycles with improvements such as intercooling, heat recovery by regenerator and blade cooling using steam are studied. The different cycles are compared with the combined cycle and the intercooled regenerative cycle. The conclusion is that the steam injected cycles have high efficiency and specific power. Adding heat exchangers to the cycle is not beneficial. Using steam as coolant for the blades offers interesting perspectives. Copyright

Thermal hydraulic study of a single row heat exchanger with helically finned tubes

In this study, the heat transfer and friction correlation of a single row heat exchanger with helically finned tubes are experimentally determined. The transversal tube pitch was parametrically varied. A detailed description of the test rig and the data reduction procedure is given. A thorough uncertainty analysis was performed to validate the results. The proposed heat transfer correlation can describe 95% of the data within +/- 11% and shows a 4.49% mean deviation. The friction correlation predicts 95% of the data within +/- 19% with a mean deviation of 6.84%. The new correlations show the same trend as most correlations from open literature, but none of the literature correlations are able to accurately predict the results of this study.

Performance prediction of compact fin-and-tube heat exchangers in maldistributed airflow

An experimental study of a fin-and-tube heat exchanger was performed in two different configurations (single and three-screen mode). To this end, a test rig was constructed to evaluate the heat transfer capacity on the air side and water side. A wide range of Reynolds numbers on the air side was investigated. A series of measurements was performed with uniform inlet flow conditions. These served to determine the heat transfer correlation for the fin type using the Wilson plot method. No correlation was available, as the fin is an adapted inclined louvered type. To validate these results, a thorough uncertainty analysis was performed. Parallel to the experiments, a simulation program was written, designed to take non-uniform flow into account. The program is based on a local (section by section) analysis scheme. To validate the program, a series of non-uniform measurements was performed. Results showed that the program is able to predict the impact of non-uniform inlet flow conditions. The numerical code can be used as a design tool to develop more efficient heat exchangers.

Plasma vitamin A in haemodialysis patients

Different high performance liquid chromatographic systems were applied to the investigation of vitamin A metabolism in subjects undergoing haemodialysis. Plasma levels of retinol, retinyl esters and retinoic acid were measured. There was a significant elevation of plasma retinol and dialysis failed to normalise this level. No correlation with plasma concentrations of creatinine or urea was found. No differences in retinyl ester and retinoic acid levels were observed between healthy subjects and haemodialysis patients. These results suggest that retinol accumulation is not caused by a deficiency in its oxidative metabolism.

Update on single-screw expander geometry model integrated into an open-source simulation tool

In this paper, a mechanistic steady-state model of a single-screw expander is described with emphasis on the geometric description. Insights into the calculation of the main parameters and the definition of the groove profile are provided. Additionally, the adopted chamber model is discussed. The model has been implemented by means of the open-source software PDSim (Positive Displacement SIMulation), written in the Python language, and the solution algorithm is described. The single-screw expander model is validated with a set of steady-state measurement points collected from a 11 kWe organic Rankine cycle test-rig with SES36 and R245fa as working fluid. The overall performance and behavior of the expander are also further analyzed.

Heat Exchanger Behavior in Non Uniform Flow

An experimental study of a fin-and-tube heat exchanger was performed. To this end a test rig was constructed to evaluate the heat transfer capacity on the air side and the water side of the exchanger. The test rig and the measurement technique were devised for non uniform inlet flow. The resulting data was used to study the impact of the non uniform flow on the heat exchanger performance. Different types of non uniform flow were studied over a range of Reynolds numbers. The fin type present in the heat exchanger used in this research is an adapted inclined louvered type.

Results of urometabolic evaluation in 127 patients with renal calculous disease

One hundred twenty-seven selected stone formers were evaluated. With the simple ambulatory tests proposed by Pak for metabolic screening and a complete urologic evaluation an anomaly was found in more than 90 per cent of the cases. The patients were divided into three groups: (1) patients operated on for staghorn stones; (2) patients with episodes of spontaneous stone eliminations or young people with only one stone episode; and (3) patients operated on for stone disease. No significant differences were noted except for the occurrence of urinary tract infection and for struvite and calcium oxalate occurrence in the different groups. Urinary tract infection combined with a metabolic disorder appear to make the evolution from small kidney stone to staghorn stone a reality.