Tao Wenquan

Refinement of the convective boundedness criterion of Gaskell and Lau

Based on the normalized variable diagram, the weakness of the Gaskell and Laus convective boundedness criterion (GL‐CBC) is revealed by numerical example. By careful consideration of the smoothness of the normalized variable variation pattern, more rigorous constraints on the interface value interpolation are found. A new CBC is thus proposed, whose feasibility and correctness are demonstrated by the inspection of ten existing bounded schemes and a numerical example.

Numerical simulation of convergent and divergent tapered pulse tube cryocoolers and experimental verification

In this paper, two models were introduced with the governing equations to simulate two kinds of tapered pulse tube cryocoolers. The simulation results were in good agreement with the results of respective experiments. It was also found that there was an optimum cone angle for tapered pulse tube cryocooler. When the cryogenic refrigerator works with this optimum cone angle, the performance could be improved a lot, which means decreasing the lowest cooling temperature and/or increasing the cooling power.

Gas-side fouling, erosion and corrosionof heat exchanger for middle and low temperature flue gas waste heatrecovery

With the ongoing emphasis on the efficient use of energy, much effort is put on the development and application of various heat recovery technologies. Heat exchangers play an important role in energy transfer and utilization and are widely used to recover waste energy from different kinds of heat sources in many fields such as energy and power engineering, petroleum and chemical engineering, air conditioning and refrigeration, aviation and aerospace, etc. In the heat exchangers for waste heat recovery, the gas-side fluid mediums usually are unclean and corrosive. The problems of fouling, erosion and corrosion of heat exchangers in the middle and low temperature flue gas-side caused by the high ash content, viscous and corrosive components are inevitable, and how to effectively solve the problems has been the subject of much research in recent years. In this paper, the researches of fouling, erosion and corrosion characteristics in middle and low temperature flue gas heat exchangers are reviewed. The causes and influence factors are discussed. Firstly, the fouling mechanisms of heat exchangers are introduced and the existing theoretical models, empirical models, and numerical methods of particle transport, impact and deposition are reviewed. The ash fouling characteristics of 6-row tube heat exchangers are presented and the effects of flue gas velocity, particle diameter, spanwise tube pitch, longitudinal tube pitch, tube shape and geometry conditions on fouling rate are discussed. Both decrease of the longitudinal tube spacing and increase of spanwise spacing can effectively reduce the deposition of particles. The geometry of elliptical tubes can fulfill this requirement for fouling reduction with smaller longitudinal spacing and larger spanswise pitch. The feasibility of using oval tube is explored for the fouling reduction. Compared with the circular tube arrangement, the deposition rate of the elliptical tube was decreased by 68.8%. Secondly, the existing erosion mechanisms, theories and methods are introduced. Different erosion models, including the model advantages and disadvantages are also summarized. The dynamics behaviour of the entrained solid particles in the flow and the deformation of the tube surface due to erosion are presented. The effects of particle diameter, particle mass flow rate and fluid Reynolds number are discussed. Among the three parameters, fluid Reynolds number has the most important effect on the erosion of tube surface. Erosion mainly occurs at the first part of the single tube surface. Thirdly, the existing mechanisms, theories and methods of corrosion characteristics of heat exchangers are introduced. Then a coupling numerical model, developed in author’s previous research, is discussed to predict the condensation characteristics of sulfuric acid vapor on heat exchanger surfaces. The corrosion risk can be reduced by decreasing the water vapor concentration and increasing the flue gas temperature. The effects of operating parameters and geometry conditions on the sulfuric acid condensation and corrosion characteristics are also discussed. A correlation of Sh number of sulfuric acid versus fin geometries for a 10-row tube bundle is provided. In order to reveal the anti-corrosion characteristics of different materials and the coupling mechanisms between the fouling and dewpoint corrosion, the online water-cooled test system and laboratory static sample test system on the corrosion characteristics of heat exchangers are introduced. Some dewpoint corrosion resistant steels are presented. The mechanism of dewpoint corrosion is revealed, the relationship between the thickness of corrosion layer and wall temperature is discussed and the coupling mechanisms between the fouling and dewpoint corrosion are also discussed. The corrosion products are composed of the ash and acid reaction products in the outer layer, iron sulfate in the middle layer, and iron oxide in the inner layer. The innermost layer is the main corrosion layer. Finally, the research needs and prospect of fouling, erosion and corrosion research are discussed. Hope to solve the problems of designing the anti-fouling, anti-erosion and anti-corrosion heat exchangers, and promote the development of efficient utilization technologies in middle and low temperature flue gas heat exchangers.

The effect of tube wall heat conduction on the natural convection in a long cylindrical envelope with an adiabatic lateral surface

The effect of the tube wall heat conduction on the natural convection in a tilted long cylindrical envelope with constant, but different temperature of the two ends and an adiabatic outer surface was numerically investigated. The envelope is supposed to be a simplified model for the pulse tube in a pulse tube refrigerator when the pulse tube is positioned at different orientations. It is found that the cylindrical envelope lateral wall heat conduction can enhance the heat transfer from the hot end to the cold end, not only because of the increase in pure heat conduction in the wall, but more importantly, also the intensification of the natural convection within the enclosure. This enhancement is resulted from the big temperature difference between the tube wall and the adjacent fluid near the hot and cold ends. Adoption of low thermal conductivity tube can effectively reduce such additional heat transfers from hot to cold end, thus reducing the loss of cooling capacity for the pulse tube refrigerator.

Numerical investigation and analysis of heattransfer enhancement in channel by longitudinal vortex based on fieldsynergy principle

Abstract3-D numerical simulations were presented for laminar flow and heat transfer characteristics in a rectangular channel with vortex generators. The effects of Reynolds number (from 800 to 3 000), the attack angle of vortex generator (from 15° to 90°) and the shape of vortex generator were examined. The numerical results were analyzed based on the field synergy principle. It is found that the inherent mechanism of the heat transfer enhancement by longitudinal vortex can be explained by the field synergy principle, that is, the second flow generated by vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient. The longitudinal vortex improves the field synergy of the large downstream region of longitudinal vortex generator (LVG) and the region near (LVG); however, transverse vortex only improves the synergy of the region near vortex generator. Thus, longitudinal vortex can enhance the integral heat transfer of the flow field, while transverse vortex can only enhance the local heat transfer. The synergy angle decreases with the increase of Reynolds number for the channel with LVG to differ from the result obtained from the plain channel, and the triangle winglet performs better than the rectanglar one under the same surface area condition.

Three-dimensional numerical simulation of thebasic pulse tube refrigerator

A three-dimensional physical and numerical model of the basic pulse tube refrigerator (PTR) was developed. The compressible and oscillating fluid flow and heat transfer phenomenon in the pulse tube were numerically investigated using a self-developed code. Some cross-section average parameter variations such as velocity, temperature and pressure wave during one cycle were revealed. The variations of velocity and temperature distributions in the pulse tube were also analyzed in detail for further understanding of the working process and refrigeration mechanism of PTRs.

Numerical analysis on heat transfer enhancement by longitudinal vortex based on field synergy principle

Three-dimensional numerical simulation results are presented for a fin-and-tube heat transfer surface with vortex generators. The effects of the Reynolds number (from 800 to 2 000) and the attack angle (30° and 45°) of a delta winglet vortex generator are examined. The numerical results are analyzed on the basis of the field synergy principle to explain the inherent mechanism of heat transfer enhancement by longitudinal vortex. The secondary flow generated by the vortex generators causes the reduction of the intersection angle between the velocity and fluid temperature gradients. In addition, the computational evaluations indicate that the heat transfer enhancement of delta winglet pairs for an aligned tube bank fin-and-tube surface is more significant than that for a staggered tube bank fin-and-tube surface. The heat transfer enhancement of the delta winglet pairs with an attack angle of 45° is larger than that with an angle of 30°. The delta winglet pair with an attack angle of 45° leads to an increase in pressure drop, while the delta winglet pair with the 30° angle results in a slight decrease. The heat transfer enhancement under identical pumping power condition for the attack angle of 30° is larger than that for the attack angle of 45° either for staggered or for aligned tube bank arrangement.