compact tubular ceramic heat exchangers for micro gas

Compact tubular ceramic heat exchangers for micro gas turbine engines

A. V. Soudarev, A. A. Souryaninov & B. V. Soudarev Boyko Research-Engineering Ceramic Heat Engines Center, Russia

Abstract

Skid-mounted power plants have been finding ever-widening application on the power generation market [1]. Flexibility, compactness and the light weight of small GTEs promote their use as the main components of such power generating plants. However, to increase their cost-effectiveness, a notable elevation of the working media parameters is a must. What constrains the above elevation is the problems of reliability and durability of operation of hot parts of the turbine which need to be cooled but, first and foremost, it is a degradation of the effectiveness of the small-size blades for compressors and turbines. Furthermore, at turbine cooling, a power is consumed which is connected with heat removal and, also, the power consumption for treatment and circulation of the cooling agent tends to increase as well. This results in an extra lowering of the level of the temperature increase effectiveness and restriction of the critical cost-effectiveness of a GTE. Findings of the convective heat transfer and hydraulic resistance studies in the internal path of the path of the high-temperature gas ceramic cooler are presented. It was demonstrated that the application of the combined technique of heat exchange enhancement using a string of ball-turbolators separated by fin inserts allows an increase in the compactness factor for the heat exchanger matrix up to 400-600 m2/m3. Keywords: ceramics, heat exchange, micro gas turbine engine, ball turbolator, internal fin insert, finning.

1 Introduction

An actually complete solution of all the complications linked with the growth of the working media parameters for small gas-turbine plants might be provided by application of the ceramic structural materials (SCMs) [2].

é Heat Transfer VIII, B. Sund n, C. A. Brebbia & A. Mendes (Editors)© 2004 WIT Press, www.witpress.com, ISBN 1-85312-705-1

In this case, there is no need in cooling of the high-temperature parts of the flow passage of the gas turbines which reduces the power consumption on the GTE’s own needs dramatically. At the same time, an increase in the initial gas temperature results in an increase in its temperature downstream of the power turbine to such level when application of heat-resistant alloys and steels to heat exchangers meant for heat regeneration becomes practically inadmissible as they do not ensure a required durability. The temperature of the discharged gases is likely to amount to 1000°C and higher. Only parts made of SCM can operate reliably at such temperatures [1]. In connection with the above-stated, SCMs should be applied to protect not only the heat-loaded parts of gas turbines but, also, the design elements of GTE heat exchangers, in particular, air-heaters operating at high gas temperatures and a high heat regeneration ratio [3]. It is known that SCMs, besides their unique positive properties determining their high heat resistance, corrosion and erosion resistance, have a number of negative properties as well. This is first an essential effect of the scale factor on such characteristics of SCMs as strength thermocyclic loads resistance, etc. So, a desire to have the compactness of the ceramic heat exchangers (HE) encreased is due not only to need in improvement of their mass-size characteristics but, also, to requirement to increase their operation reliability with the reliability increase requirement being of the primary importance. In connection with the above, the high-temperature HEs are made as double-sectional, i.e. including ceramic and metal sections. With use of such HEs, the ceramic section is made as small as possible. This section serves mainly to cool the high-temperature gas to a level admissible for application of cobalt and nickel alloys and, also, austenitic heat-resistant steels. An improvement of MSC for a ceramic section could be achieved by a heat exchange enhancement through both the convective and radiant components of the convective heat transfer coefficient.

2 Heat exchange enhancement in ceramic GC

To manufacture the matrix of the ceramic HE, tubular HEEs are widely applied [4]. Increasing the heat power of such HEEs is achieved by heat exchange enhancement both inside and outside tubes. The latter can be implemented using a cross or longitudinal finning (Fig.1). The like finning could be also applied inside the HEE channels though it needs an ideal contact of the fin with the tube wall. This is achievable only when parts made of SCM are welded one to another [5], [6]. Or hydraulic turbolators as a string of ball elements placed in the core of the gas stream along the channel axis can be used [7]. As they are flowed around, the gas stream is squeezed towards the wall of the channel, its velocity increases, the convective heat transfer increases too. The effective thermal conduction of the gas path also increases. Furthermore, the ball elements flowed around by gas do not only agitate the gas stream but, being heated to the temperature close to the gas


254 Heat Transfer VIII

temperature, release some heat to the “cold” wall of HEE by radiation. All these measures promote increasing the specific heat output of HEE [8]. Study of the heat output of the tubular HEEs at different flows of the working media was carried out on the sub-scale GC (Fig.2).

Figure 1: Tubular heat exchange elements for ceramic gas cooler. Tubes with cross (a) and longitudinal (b,c) outer finning with fin (b) and combined (c) turbolators; matrix for the GC models of seven (d), nineteen (e) and twenty five (f) finned tubes; GC module of nineteen finned tubes.

Figure 2: Working area with the 7-tube model of ceramic GC in each tube

thereof a turbolator in the form of a string of ceramic balls is installed. 1 – tubular matrix; 2 – casing; 3 – pusher; 4 – perforated grates; 5 – string of ceramic ball-turbolators; 6 – air supply (discharge) pipings; 7 – static pressure bleeding tubes.


Heat Transfer VIII 255

The model matrix consisted of 7 finned ceramic tubes with turbolators as a string of balls inside their inner space. The balls were placed on the Ni-Cr alloy wire and separated by thin-wall 2 mm diameter tubes made of stainless steel. The tests were carried out on the test bed (Fig.3) at the gas temperature at the model inlet tg/=900-1,100°C, and that for air is 40-60°C.

Figure 3: Schematic diagram for test bed for «hot» tests of the GC matrix

model. 1 – model; 2 – discharge piping; 3 – combustor;4 – air supply; 5 – orifice plates; 6 – fan.

Gas moves inside tubes whereby flowing around the string of the ball elements while air moves outside as a counterflow towards the gas stream along the slot channels formed by the external longitudinal tube finning. The heat transfer agent flows across each tube do not exceed 0.8-2.2 g/s. The test results are compared with the data of calculations (Fig.4) performed with use of the criteria equations by Burdanov [9] (internal space of TEs) and Petukhov [10] (intertube space of model).

Figure 4: Heat transfer in ceramic model of GC. 1 – test data; 2 – calculation

with use of Burdanov’s [9] and Petukhov’s [10] formulae; 3 – same with the radiant component of the heat transfer coefficient accounted for.



It can be seen that all the test points are 10-12% above the calculation relationship for the coefficient of convective heat transfer from the heat transfer agent (line 2). The reason of the above is that at calculating of the coefficient of convective heat transfer the heat transfer by radiation from the heated balls through the transparent gas stream to the cooled wall of TE was not accounted for. A refinement of the methodology of identification of the coefficient of convective heat transfer inside the tube in the form of his= hcon,is + hα,is, (1) where

wÒblÒ

wÒblÒ

red,is,

h−

−

α=α

4

100

4

100675 (2)

w.SA

bl.SA

wbl

red

−

α+

α

=α

111

1 (3)

provided the best agreement for the design and test data (Cf. the test points 1 and line 3). Along with the growth of the heat output of TE at the heat exchange enhancement using the string of balls, the hydraulic resistance of its inside path increases notably, too. The discrete location of the balls in the string with a streamlined configuration of the channel, production of conditions favourable for the balls’ levitation, all these measures promote a reduction of the pressure losses in the internal path of TE whereby increasing its power effectiveness. Though, with the discrete location of the ball-turbolators, the transfered heat flux decreases. To achieve a partial recoupment for the above decrease, one can use a combined turbolator (Fig.1) where balls are separated by ceramic fin inserts. The diffusive welding of fins to the inside wall of the tube will ensure an ideal thermal contact. Increasing the inside surface of the tube (depending on the number of fins on the insert) can be 20-50%. The separating inserts in the high-temperature stream, like the balls, release some heat by radiation to the tube wall. So, the formula for the calculation of the reduced inside coefficient of convective heat transfer at the areas of location of the fin inserts [11] was thus refined. It must contain the radiative component



α+ψ+= αT

conwconw

confinfin

fullS

finS

fullS

bfSconwred

h

h

h

h

A

A

A

Ahh

..

.

.

.

.

.,

1,11,1

(4)

where AS.bf, AS.fin, AS.full are the areas of interfin surface of wall, fins and full area of internal surface of finned tube

AS.full = AS.bf + AS.fin . (5)

To calculate the coefficients of convective heat transfer along the triangular channels formed by the inside fins and tube wall, the empirical formula by Mighay [12] with correction for the relative length of the fin is used.

.

/

finL

eqD

L

32

1

+=ε (6)

3 Hydraulic resistance

The major contributor to the hydraulic resistance of the channel with the combined turbolator as a string of balls separated by fin inserts is the ball elements. To calculate their hydraulic resistance with the air moving along the round section tube, the following equation of similarity can be employed:

,

zK

wD

wL

.

.

.

blD

wD

.Re.smEuw

bl

D

D3822

66701

351

240014

−

−

−−⋅=

(7) 42.3)/(28.1 −−

=

blDwD

blZ

D

ZK (8)

1,067 ≤ Dw/Dbl ≤ 1.39; 1 ≤ Z/Dbl ≤ 22.2.

In this formula the Euler’s and Reynolds’ numbers are identified by the gas velocity Usm in “hollow” tube. With the separating fin baffles available, the pressure loss across the inside path of TE increases (Fig.5).



Figure 5: Hydraulic characteristics of ceramic tubes with combined turbolator.

The hydraulic characteristics of the ceramic tubes with the strings of balls separated by inserts with the various number of fins 0 (Eu=Esm), 4, 8 and 13 are shown in this figure. It is evident that the more number of tubes the higher is the hydraulic resistance of the path

Eu = KA Eusm. (9) In this formula, the coefficient KA depends on the coefficient of development afin for surface of wall of the internal channel. For the criss-cross inserts KA=1.53 and for the fan-shaped inserts with 8 and 13 fins, the following formula can be employed:

KA=0.736 exp (0.296 afin), afin = AS.fin/AS.w (10)

4 Conclusions

Thus, the application of the combined heat exchange intensifiers in the form of a discrete string of ball elements separated by fin inserts with some heat of the high-temperature gas stream by radiation to the internal channel wall accounted for, despite a substantial increase in the hydraulic resistance, allows increasing the power effectiveness of TE and its compactness (χ=400-600 m2/m3).



Abbreviations

AH – airheater GTE – gas-turbine engine GTU – gas-turbine unit SCM – structural ceramic material MSC – mass-size characteristics GC – gas cooler HE – heat exchanger HEE – heat exchange element

Nomenclature

Deq – equivalent diameter of interfin channel KT – correction considering non-uniformity of temperature distribution across radiant fin Lfin – fin length lengthwise gas stream motion Z – ball spacing in string αred – reduced emissivity factor for “ball-wall” system εL – correction for relative fin length χ - compactness factor ψfin – fin effectiveness factor

Indices

a – air bf – interfin bl – ball con – convective eq – equivalent fin – fin full – total is – inside red – reduced sm – smooth w – wall, tube α - radiation

References

[1] Colin F. McDonald. Ceramic heat exchangers – the Key to high efficiency in very small gas turbines. ASME Paper 97-GT-463, 12p.

[2] Soudarev A.V. Prospects of production of environmentally friendly ceramic gas turbine engines for stationary power. News RAN, Power, v.38, N 1, , pp.49-59, 1992.



[3] Soudarev A.V., Tikhoplav V. Yu., Ceramic Heat Engines (NIZ KTD) at the Research-Technological Institute for Power Engineering (NITI EM), edited by Mark van Roode, Mattison K., Ferber, David W. Richerson, Gas Turbine Design and Test Experience, Progress in Ceramic Gas Turbine Development, ASME PRESS, , vol.1, ch.32, pp.683-707, 2002.

[4] MacDonald. Outlooks for application of ceramic heat exchangers for power and raw materials saving. Proc. AOIM, Power mashines and plants, , v.102, N 2, pp.69-87 (translation from the ASME Journal), 1980 .

[5] Soudarev A.V., Grishaev V.V. Ceramics Sugrav Applied to High Temperature Path of 2.5 10 MW GTU. Presented at 1996 Kongress und Ausstellung fur Werkstoffe und Auwendungen, 28-31.05, , Stuttgart Werkstoff Woche’96, Symposium 3, pp.195-200, 1996.

[6] Soudarev A.v., Grishaev V.V., Structural Alumino-Boron-Nitride Ceramic Material for Gas Turbine Engine Applications, by Mark van Roode, Mattison K., Ferber, David W. Richerson, Gas Turbine Design and Test Experience, Progress in Ceramic Gas Turbine Development, ASME PRESS, , vol.2, ch.13, pp.245-259, 2003.

[7] Westbrock A.J. Turbolator. Patent of USA, N 3.921.711 filed 25.11.75 F28F13/0.8.

[8] Soudarev A.V. et al. Convective Heat Transfer and Hydraulic Resistance at Air Flow through ceramic tubes with Heat Exchange Intensifiers. Fourth International Symposium on Experimental and Computational Aerothermodynamics of Internal. August 31 to September 2, , Dresden, Germany. Flows Proceedings vol.II, p.208-216, 1999.

[9] Burdanov N.G. Researches into hydrodynamics and heat exchange in passages with ball backfill, Autoreport for maintaining of Ph/D degree, Moscow, , 16 p, 1980.(in Russian).

[10] Petukhov B.S. Heat exchange and resistance at laminar flow of liquid across tubes. M., Energhia, , 412 p., 1967 (in Russian).

[11] Krivenko A.A., Chernyakov A.G. Convective heat transfer and resistance of tubes with internal discontinuous finning. Heat and Mass Exchange – MMF-92, v.1, Part 1, Minsk, , pp.125-128, 1992. (in Russian).

[12] Mighay V.K. Increasing the effectiveness of the existing heat exchangers. L., Energhia, 144 p, 1980 (in Russian).



compact tubular ceramic heat exchangers for micro gas

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