Exploitable results
The four main results deriving from the research project can be summarised as follows.
1) Novel heat exchanger tube bundle design for reduced fouling rates and augmented heat transfer efficiency (DDEFORM tubes)
A major problem in lignite utility boiler heat exchangers is the extensive and rapid fouling of the heat transfer surfaces of the heat exchanger tubes which inevitably leads to a reduction in their heat transfer efficiency. The novel tube bundle design resulting from the present research project reduces the fouling rate and pressure drop so that the number of tubes in the heat exchanger can be doubled by reducing the transverse spacing. The overall performance reaches a 90% increase in heat transfer at 25% of the fouling rate and 60% of the pressure drop of the standard configuration currently in use by the Public Power Corporation of Greece.
However, the configuration is not restricted to lignite utility boilers but may be applied to any type of heat exchanger tube bundle that operates under potentially fouling conditions or where pressure drop becomes an important factor. Heat exchangers used for cooling (i.e. automobile radiators) are another market sector in which the present results could be applied.
The DDEFORM tubes have been patented by the NTUA.
2) Extended knowledge and relative efficiencies of the use of active and passive techniques for heat transfer augmentation and fouling minimisation in heat exchanger tube bundle configurations.
Throughout the present project, a number of active and passive techniques have been investigated, targeting heat transfer augmentation and fouling minimisation in lignite utility boiler heat exchangers. Upstream flow perturbations, asymmetric arrangements and non-circular tubes were numerically and experimentally investigated leading to significant findings regarding the relative efficiency of these techniques in optimising heat exchanger tube bundle efficiency. It is obvious that the relative effect of the various techniques on heat exchanger efficiency can be extrapolated to almost any heat exchanger tube bundle application and not only those of lignite utility boilers. Heat exchangers used for cooling as well as heating can be included in the field of relevance. To the partners knowledge there have not been any other extensive comparisons of these techniques published in the literature. The studies are therefore of important scientific impact, having direct relevance to the study of heat exchanger operation and design procedures.
The technological know-how has been attained throughout the project using both numerical and experimental procedures by Kings College and the NTUA. Both partners are willing to share the knowledge through further collaborations in the form of consulting, research or demonstration.
3) Experimental facilities for investigating gas-particle flows.
GRETh has a general expertise in fouling and in experimental investigation, and more specifically in the field of industrial heat exchanger. During the present contract, we have used this expertise to build a new experimental loop and measure the particle deposition on a cylindrical in-line tube bundle and on an enhanced in-line tube bundle. We have also extended our expertise to the thermophoretic deposition inside heat exchangers. The potential use of these results is short term exploitation and long term exploitation. For the short term, the direct exploitation by heat exchanger manufacturer using the new enhanced tube. For the long term exploitation, the experimental loop and methodology can be applied to other heat exchangers or filter geometry for optimisation.
The results of the work consist in an extension of our expertise in particle deposition measurements and fouling mitigation. We will be able in the future to test heat exchangers and filters in perfectly controlled fouling conditions and to suggest to heat exchanger manufacturer new tubes-shape to reduce the fouling. The main advantage of the new loop is to provide reliable results in actual conditions and in short time compared with classical fouling tests.
The results obtained in the contract have many other applications. The loop and expertise developed here can be extended to other technology where it is essential to control and measure the particle deposition i.e. filters application for example.
4)Experimental facilities for investigation of steady and unsteady flows past heat exchanger tube bundles or other bluff bodies.
Kings College London has extensive expertise in the application of optical diagnostic techniques in the study of a number of flows of engineering interest. A previous JOULE contract allowed KCL to apply these techniques in the study of steady flows past tube bundle models typical of those used in the lignite boilers of the Public Power Corporation. The present contract built on the expertise and the experimental facilities that resulted from that contract. Thus, the facility was modified to allow pulsating (unsteady flows) to be studied and a number of novel tube bundle arrangements were constructed. This facility was severely damaged in the course of the investigation and had to be replaced. The expertise of KCL was significantly strengthened by the present contract particularly in the area of unsteady flows, and it was used for the design of a new state-of-the art experimental facility. It is envisaged that this facility will allow researchers at KCL to extend their activities into the study of steady and unsteady flows past bluff bodies, the study of turbulence and a number of other engineering applications of industrial relevance.
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