An expert system for convective heat transfer measurements using a transient analysis

An expert system for convective heat transfer measurements using a transient analysis Ali R. Uzel, Richard J. Edwards and Bryan L. Button Department of Mechanical Engineering, Trent Polytechnic, Nottingham NG 1 4BU, UK (Received February 1989)

ABSTRACT Expert systems have been implemented in many disciplines. Engineers too have found them useful for several applications ranging from engine fault diagnosis to the design of components. This work describes a system called 'HEATEX' which provides advice on how to make measurements of local convective heat transfer coefficients using a transient technique. The tool has numerous applications in assisting the design of components requiring a knowledge of heat transfer characteristics. INTRODUCTION Method

The experimental method determines the local heat transfer coefficient by means of an analysis of transient wall heating. It uses the measurement of time required for the surface temperature of a test specimen to attain a predetermined value when subjected to a heated flow field. This data, along with the thermal properties of the test specimen material, is used in a transient analysis which yields local convective heat transfer coefficients 1. The surface temperature of the test specimen is obtained by thermal indicators of which the three major types are: irreversible phase change paints; reversible colour change paints and reversible microencapsulated thermochromic liquid crystals. The choice of thermal indicator is dependent upon required repeatability, resolution, accuracy and fluid temperature. The analysis requires the coated test specimen to be subjected to a step increase in flow field temperature. This can be achieved, for a thermal wind tunnel (Figure 1 ) by introducing the specimen to the heated flow field by swiftly rotating the specimen holder about its fulcrum from the upright position until it is 0952-1976/89/010040-0952.00 © 1989 Pineridge Ltd

40

Eng. Appli. of AI, 1989, Vol. 2, Match

parallel to the flow. Cool air of known temperature is passed through the specimen prior to the experiment in order to obtain the test specimen's initial temperature. Once the test specimen has been introduced into the heated flow field, the progression of the transformation line (Figure 2) of the thermal indicator is recorded by a video camera/recorder or a 35 mm camera, depending on the application. Two items of data obtained from the output of the recording device are the position of the thermal indicator's transformation and the time taken to reach that position. Local heat transfer coefficients are then determined by the solution of the transient heat conduction equation by numerical methods. When insulative specimens are used (e.g. perspex) the heat transfer situation within the specimen can be treated as one-dimensional, otherwise a twodimensional case is considered. Potential of the method

The experimental method is established as a novel technique of determining rates of convective heat transfer• Numerous geometries in a wide range of flow fields have been investigated by engineers in various fields. NASA design engineers x used the technique to determine heat transfer characteristics and temperature distributions of the space shuttle in supersonic flow. Ireland and Jones 2'3 have undertaken extensive work with the technique to design cooling passages within turbine blades. Jambunathan et al. 4 carried out an analysis and validation of the technique for the case of natural convection on a flat plate. Edwards 5 investigated heat transfer phenomena in entrance regions of smooth annular ducts fitted with longitudinal fins and swirling flow. Due to the technique's versatility, the range of application is vast. Unlike many experimental heat/mass transfer techniques, the method is essentially straightforward and yields full field data with high accuracy and resolution. The equipment costs are low, results can be obtained relatively quickly, and qualitative results can be obtained without mathematical analysis. These factors make the technique a useful tool for an industrial development environment rather than a research laboratory• Jambunathan et al. 4 and Edwards 5 have identified several experimental parameters which greatly affect the accuracy of the technique. An expert system has been developed, based on the results of this work and the optimal criteria proposed. In order to cut system development time to a minimum, the shell approach 6 was used.

An expert system for convective heat transfer measurements: A. R. Uzel et al.

FLOW

I( I -~]

~

SideElevo¢"Io¢~

Figure I

Thermal wind tunnel assembly

SYSTEM D E V E L O P M E N T It was decided to develop a system which would assess the feasibility of conducting tests using transient methods and if satisfactory, would continue to give advice on the equipment required, specimen parameters and test procedures. An expert system shell called 'Savoir '~ was used to develop HEATEX.

The different ways in which the user can answer questions are given in Reference 6. It was found that the use of 'real', 'integer', 'string' and 'conditional' types of user-input data types was sufficient for this "application. 'Certainty scales', which accommodate user judgement and are generally used for diagnosing faults or infections, estimating likelihoods or making predictions, were considered unsuitable for this work.

Application domain HEATEX's reasoning and the rules contained in its knowledge base depend on several factors. The heat transfer situation (i.e. natural or forced convection), type of fluid, temperature, desired accuracy and repeatability are only some of many factors which the system takes into account. If the fluid is water, its dissolving effect on the thermal indicator will have to be taken into account. On the other hand, it may be possible to place the thermal indicator on a side of the specimen that is not exposed to water or to coat it with a protective layer. Alternatively, air could be used and the similarity with the original geometry could be achieved using dimensional analysis. As well as trying to find out if any one of these solutions is acceptable to the user, and their order of importance, the system has to know in advance, for example, which thermal indicators are affected by fluids and to what degree. HEATEX makes its deductions from similar facts and their relationships using the evidence obtained from user replies. As there are many factors which have to be taken into consideration, numerous combinations of advice can be obtained.

Capability HEATEX's advice on the equipment required includes the material of the test specimen (unless the tests can be carried out on the actual component); the experimental set-up (e.g. a wind tunnel with an appropriate specimen-mounting mechanism); an appropriate thermal indicatorS'9't°; a suitable recording facility and a computer algorithm if necessary. For example, for the thermal indicator recommended, HEATEX can decide if calibration is necessary, suggest a suitable technique and give information on suppliers and storage requirements. If the desired accuracy is relatively low, a simple fan and heater instead of a thermal wind tunnel can be recommended for the application. After advice on equipment for the application has been given, the system suggests a test procedure. This includes the preparation of the specimen, application of the thermal indicator, introduction of the specimen to flow field, recording of the data and its analysis. Depending on the type of material selected, HEATEX recommends a suitable numeriEng. Appli. of AI, 1989, Vol. 2, March

41

An expert system for convective heat transfer measurements. A. R. Uzel et al.

Elapsed time=87.6 s

Elapsed time=70.3 s

Figure 2

Elapsed time= 122.3 s

Progression of thermal indicator transformation time in annular duct flow

cal algorithm, unless the user requires visual mapping only.

Consulting HEATEX H E A T E X is a turn-key system, easy to load and consult. As with many other systems, it has an initial menu providing three options: to consult the system,

42

Elapsed time = 164.5 s

Eng. Appli. of AI, 1989, Vol. 2, March

to exit from the system or to access the 'introduction menu'. The latter contains options to obtain instant help or information on H E A T E X and expert systems in general, or to return to the initial menu. The descriptions of HEATEX, its aims, assumptions, benefits and limitations, together with background information on the heat transfer method and the thermal indicators, are provided. As with learning

An expert system for convective heat transfer measurements: A. R. Uzel

f

I .DOMAIN I

J INDEPENDENTL / ~

et al.

,t

I -

/

DOMAIN INDEPENDENT ~ i

i

uel # 7

j

Figure 3 Program classification

material from a book, the mechanism for forward and backward paging is provided. Once the option for consulting the system is selected, aquestion-and-answer session begins. The system's questions can range from a minimum of 6 to a maximum of 18, depending on the application. When HEATEX is in the question mode, the user can interrogate it in a number of different ways as well as answering a question. The interrogation facility enables the user to ask for help or further explanation of the question, to investigate the line of reasoning employed by the system, to backup to the previous question or to store the consultation on a hard-copy device. While the user answers questions, the system's reasoning mechanism analyses the evidence accumulated so far, in order to determine the next appropriate question. If possible, a report is generated which eventually constitutes the bulk of the system's advice. Comprehensive advice is given only if it is feasible for the system to recommend a test procedure, otherwise there is an explanation on 'why not?' as well as some advice on 'what to do next'. System structure

The system consists of a number of individual programs, written in the authoring language of the Savoir shell, where any one of them can be used as

part of another system if required. The programs can be divided into two main groups; domain-dependent and domain-independent (Figure 3). The domaindependent programs contain knowledge related only to the application, which in this case is experimental heat transfer. The domain-independent programs manage the consultation, the menus and the standard help facilities. A program can be either "active' or 'passive'. Passive programs contain the system variables (e.g. goals and questions), frames and production rules 6. In other words they contain all the facts and their relationships (Figure 4) required by the system before any evidence can be obtained from the user. During the consultation process, all the variables within the frames and rules gradually take on final values through a pure reasoning process unless 'active' programs interrupt or alter the flow of logic. Active programs contain a series of 'action' declarations (sometimes referred to as 'demons') which interrupt or by-pass the pure reasoning mechanism of the passive programs to execute a series of actions as soon as certain specified conditions are satisfied. Some examples of actions include displaying messages, clearing of certain system variables and investigating alternative propositions. Savoir's inference engine incorporates an automatically-controlled goal/data directed search mechanism. This synchronises the pure reasoning Eng. Appli. of AI, 1989, Vol. 2, March

43

An expert system for convective heat transfer measurements: A. R. Uze/ et al.

Goal Levels Top

RCCP

[

I c°at-'c

I

I med-te'p I

I 'epeat2 I

[

case_3

I

Figure 4 A typical tree-like representation in HEATEX

within frames (Figures 4 a n d 6) of the passive programs, unless overridden by the demons. During the consultation, the advice-generation mechanism contained in an active program, gradually propagates whenever the system obtains a new piece of evidence. This continues until all the evidence is obtained. Each time a new condition (attached to a demon) is satisfied, the system selects a text block (Figure 5) from a set of alternatives associated with a level of advice. The text block (B) with the highest level number is displayed to the user first. At each level, there is a possibility that one or more smaller text blocks (c, a), which can be warning messages, reminding notes etc., can be accumulated as well as the main text blocks (B, G, etc.). This process continues until all the evidence is complete, and the relevant text blocks have been accumulated. For maximum use, the text blocks contain certain linguistic variables. For example, 'thermal_indicator_type' can become 'reversible colour change paint' or 'liquid crystal' (Figure 4).

PRACTICAL A P P L I C A T I O N The developed system was tested using the case of the enhancement of a compact heat exchanger design which is currently being investigated within the Department. The work concentrates upon the

44

Eng. Appli. of AI, 1989, Vol. 2, March

requirement to improve heat exchanger performance by increasing convective heat transfer from turbulent air flowing in the entrance region of annular ducts (Figure 7). A suitable experimental technique to obtain 'full field' data by non-intrusive means was required. Appendix A describes a consultancy with 'HEATEX' which determines whether the transient wall heating technique can be applied to this situation. Following the consultation, 'HEATEX' deduced that the transient wall-heating technique could determine the desired data. The report described how the technique could be utilised on the geometry, and advised on relevant published work. The type and manufacturer of the thermal indicator required to obtain the desired accuracy was displayed. In addition, the numerical technique required to deduce the heat transfer coefficients from the experimental results is fully explained. The system was used by various members of the Department who had little or no prior knowledge of Expert Systems or of the transient wall-heating technique. The feedback from the users was very encouraging, and little difficulty was experienced in using the software. The main conclusion drawn from the consultations was the need to expand the system to cover more techniques in order to widen its applicability, which is planned as the next stage of the project.

I

An expert system for convective heat transfer measurements: A. R. Uzel et al. Advice generated

Linguistic variable

A

/

C

B

F-

l

m

l

---

~

etc

B

r-~

I

I

I

I

1

I

C

Level K i

[

a

I

!

8

l

l

o

l

G

---

+ etc

G I/

I

I

F--'l

m I

Level

I

h

+

! etc

etc

Figure 5 Advice generation mechanism

/ (

Top gt~l

.....

......

1

Hot Air

////,/,'//

,?'."

\\\\\\\\\\\\~\

O01~)I'~ON RCCP

i-Surface under i n v e s t i g e t i on

' C o n d i t i o n o f %~X~' eas=e I OR c ~ J e 2 OR ¢=8e_3 { ....

=

L e v e l - I Igor1= . . . )

O0~ITION c~mel ' l i q u i d c r y s t l d , o s n be o ~ t e d h i g h { 9 0 t o 150 C ) ' ccet_lc_? AI~ m~l_te~

Nlaillmt ~ t e r

-~-- Cold Air

is ~ b i t

st t ~ I i m r J ~

rod trap

--~//////////////////// \\\\\\\\\\\\\\\\\~

Hot A i r

CONDITI(I~ c e s e 2 'User reqlutred r e ~ t a b L l l t y

re~eet_? ~

but t h e t ~ m p e ~ t u r e

Cold Air

between 90 t o 150 C'

CGt~ITION c~me3

{....

level 2 ,~m

........

Figure 7 Heat transfer situation

colour chsr4e l ~ t '

' t ~ e r wi~'~"m t o uoe ~ e v e r s i b l e l~/=ed_temp /~ ~_I~? }

C O N C L U D I N G REMARKS

CONDITICN med_temp ' C u e f o r low a c c u r s c y rand t e ~ e r a t u r e accurscy_23 AND temp2

l o e t ~ - - e ~ 90 t o

150 C'

CONDITION l o w / u w d ~ 'For

tec~oel~tu~ ioe~mee~ 0 t o = I) (~ ( t e B p _ ? = 2 )

150 C P

(teap_?

I cc~t_Ic ? t ~ t _ ? {....

level

tmeJ~O~_? =re i n c l u d e d i n 'i.~steR questlocm f i l e '

3 llcel= .........

}

CONDITION ~ u r k ~ - _ 2 3 'Low a ~ u - m 2 y o r t r e n d s ' (accu~cy_? = 21 OR 1 8 c c ~ , _ ? = 3 ) COULD1T ION t e m p 2 'Teml~er~ture ~ t ~ n (temb_? = 2) ( ....

level

{ @uestions fit,-' )

I ....

e=~t o f

4 go~Js

90 to 150 C' .........

}

'ac, c t x r a c y _ ? ' w t d ' t e ~ 0

g~ml K~C'P . . . . .

Figure 6

? ' l i f e i n c l u d e d i n 'syste111:Q t ~ s t i o n s

}

A typical frame in HEATEX

]

The testing of HEATEX showed that the system's recommendations are consistent with existing and proposed applications. However, the system's decision-making process is based on experience gained from applications with low temperatures, single-phase fluids and those using wind tunnels. Therefore, further refinement may be made by considering a wider range of applications. Despite the large amount of possible advice that the system can give, the real-time response on an IBM PC has proved acceptable. The HEATEX project proved that, using a shell approach, systems can be implemented fairly quickly, in this case approximately eight weeks. Eng. Appli. of AI, 1989, Vol. 2, March

45

An expert system for convective heat transfer measurements: A. R, Uzel et al.

As more-versatile shells are becoming available at competitive prices, the system development time will become shorter. This is likely to result in an increased number of potential developers. It was found that swift implementation was also due to the fact that the expert was on-site and enthusiastic about the whole project. The developers claim that the HEATEX project has enabled the domain expert not only to question and understand his own reasoning and formal knowledge better, but also to communicate ideas more effectively. They believe that, for this application, the system has more use in assisting the consultant in achieving consistency of the decision-making process than it would have in replacing him/her entirely. The system is currently being used in the laboratories as a research training tool amongst researchers who have recently undertaken projects related to experimental heat transfer. APPENDIX

A - A C O N S U L T A T I O N SESSION

WITH HEATEX Is the heat transfer situation 1) governed by forced convection? 2) governed by natural convection? 3) not yet known? (1 ... 3 or an O P T I O N followed by ) 1 Is the fluid 1) 2) 3) 4) 5) 6)

air? water or any other liquid? any fluid that dissolves perspex? a gas other than air? none of the above? not yet known?

(1 ... 6 or an O P T I O N followed by ) 1 Are the fluid temperatures experienced during the measurements within I) 30 to 9 0 C?

2) 90 to 1000 C? 3) Below 30 C or greater than 1000 C? 4) I don't know (1 ... 4 or an O P T I O N followed by ) 1 What is the desired accuracy of the coefficient of convective heat transfer? 46

Eng. Appli. of AI, 1989, Vol. 2, March

1) 2) 3) 4)

within + or - 3% within + or - 10% trends (visual inspection or mapping) sufficient I don't know

(1 ... 4 or an O P T I O N followed by ) 1 Is the experiment to be repeated more than once? (Y or N or an O P T I O N followed by Y I have now completed my investigations. I will not ask you any more questions. Next, you will be given the opportunity to select option 1 (i.e. system's recommendations) from the 'final menu'. This will give you general advice on the equipment you need to carry out the experiments and on the test procedures. The advice will depend on your replies to my questions. If you wish, you can store this advice by pressing leter 'j' followed by 'RETURN' key, then press letter 'y' followed by 'RETURN' to trigger the hard copy device ON when you see the 'final menu'. After you have finished, you must turn O F F the hard copy device pressing 'j' followed by 'RETURN' and 'n' followed by 'RETURN'. You can store any part of the consultation as long as the system is in the question mode. The consultation is stored in a file called 'log.txt'. The Report is:-EQUIPMENT NEEDED What follows from now on is a list of materials and equipment that you will require before you can start your tests. After that, you will be given a general advice on the test procedure. The following materials and equipment are required: 1) You will need liquid crystals and a coating facility (ie. a spray gun or modeller's air brush). Please note, * For high accuracy reasons, I recommend that you obtain a 'Chiral Nematic' compound type liquid crystals. They tend to be more stable both chemically and photocbemically (UV light) than cholesteric types. * Under the name 'Thermochromic Liquid Crystals', BDH Limited, Peele, England BH12 4NN,

An expert system for convective heat transfer measurements: A. R. Uzel et al.

manufacture the type of thermal indicator needed for your application. * Ensure that the fluid temperature Tb is at least by 5°C greater than the colour change temperature Tc when selecting one. * Keep liquid crystals in fridge until the coating process. 2) You need to reproduce the thermal flow field very accurately unless you intend to conduct the tests using the actual flow field. The flow and temperature control must be very accurate. Velocity and turbulence intensity must accurately represent the situation modelled. To model a thermal air flow field, you will need a thermal wind tunnel. If it is possible and costeffective to design and make a tunnel by yourselves, by all means do so. Otherwise, you will have to buy one. Whether you buy or make one, there are certain parameters you will have to consider. You will need to estimate the maximum air mass/volume flow rate for the fan/motor, and the max. power (kw) required including the losses for the .heater/motor. Also, it may be worthwhile (cost-effective for energy conservation reasons) to use a 'closed circuit' type of wind tunnel if the power requirement including the losses exceeds 35 kw for high-temperature applications. The following references may be useful for the design or the selection of the tunnel. * Button, B. L. and Tura, R. A., Design of a low speed thermal wind tunnel, Report ME 393, March 1983, Dept of Mech. Eng., Coventry Polytechnic, UK. For the evaluation of optimum experimental parameters, please consult the following reference: * Jambunathan, K. et al., Convective Heat Transfer Coefficients: The Colour Change Paint Technique. J. of Applied Energy 28 (2) 1987 3) Although the thermochromic liquid crystals are supplied with the colours precalibrated, you should check these transformation temperatures and establish if they vary from their stated value. Therefore, you will need to calibrate the liquid crystals. As the angle of illuminating light and/or the viewing angle alters, the location of the melt line could change too. So, your calibration must allow for these effects. I strongly recommend that you obtain a platinum resistance thermometer with high precision and a good mercury in glass thermometer (NPL) for the calibration process. 4) You will need a video camera/recorder system

with a recording timer of at least 0.1 s for the accuracy you require. 5) You will also need an appropriate one-dimensional heat conduction algorithm with fine meshing and a computer. The details of the algorithm are given in the references below. * Jambunathan, K. et al., Convective Heat Transfer Coefficients: The Colour Change Paint Technique. J. of Applied Energy 28 (2) 1987. * Edwards, R. J., Enhancement of Heat Transfer in Smooth Annular Ducts Using Longitudinal Fins or Swirling Flow. PhD Thesis CNAA Trent Polytechnic, 1987. 6) You will need a perspex model of your component. Perspex has a low thermal conductivity, hence the heat transfer process by conduction can be assumed to be one-dimensional. • TEST PROCEDURE What follows from now on is a general advice on the test procedure. The general procedure is outlined below. 1) SPECIMEN PREPARATION First a perspex model of the component has to be made. The size of the perspex model can be dimensionally similar to the original component. Any experiments on the specimen (i.e. perspex model) must be carried out under dynamically similar conditions to that of the original flow field. For the evaluation of optimum experimental parameters, please consult the following reference: * Jambunathan, K. et al., Convective Heat Transfer Coefficients: The Colour Change Paint Technique. J. of Applied Energy 28 (2) 1987. The specimen should be rubbed with line wire wool to remove the 'shine' from the surface so that much greater adhesion of the liquid crystal is achieved during the coating process. The specimen should then be washed to remove any debris. 2) PREPARATION OF THE THERMAL INDICATOR AND COATING SPECIMEN Spray the opposite side of the specimen matt black to improve visibility of the liquid crystals. You will have to thin the liquid crystal with one part to three of water in order to improve the spraying process. This process only alters the concentration of the micro encapsulated liquid crystals in the mother liquor, not its change temperature (Tc). Fill the modeller's air brush (spray gun) with the solution. Eng. Appli. of AI, 1989, Vol. 2, March

47

An expert system for convective heat transfer measurements: A. R. Uzel et al.

After the specimen is dry, warm it to 30°C and spray the side to be investigated (opposite side of matt-black side) with liquid crystals using the modeiier's air brush. The warming up process improves the adhesion between the liquid crystals and the test specimen. Apply about 4 thin coats and allow approximately 45 minutes for drying. Your perspex specimen has to be gridded along the regions where you wish to evaluate 'h' or 'Nu'. It would be better if the gridding was done on the video screen or on a separate transparent component specifically designed to stand between the test specimen and the camera. The spacing between the grids is entirely up to the requirements of your application. However, you are limited to a minimum spacing of 2 mm for perspex, as thermal conduction takes place between the adjacent points for distances less than 2 ram. The transient analysis requires that the specimen be subjected to an instantaneous step change of the flow field and that the initial temperature (To) of the specimen is known. One way of doing this is by means of swiftly rotating the specimen about its fulcrum from the upright position until it is parallel to the flow. At this point the timer on the video camera can be started and the progression of the melt line at temperature (Tc) can be recorded. Please ensure that: 1) The specimen is concentric with the flow by means of an adjusting device. 2) The flow field has reached thermal equilibrium at temperature (Tb) before introducing the specimen to the flow field. Note that Tb>>Tc. 3) The effects of emissivity of paint increasing local heat transfer, and the angles of illumination/view are negligible. You will be recording two items of data,

48

Eng. Appli. of AI, 1989, Vol. 2, March

the position of the colour line (d) and the time taken to reach that position (tc). From d, tc, Tb, Tc, To data, the energy balance of the equations of Fourier's Law and Newton's Law of Cooling, the specimen's thickness (t) and its thermal diffusivity, and by a computation sequential iteration technique (where by guessing values for 'h' and solving the general transient heat conduction equation for a one-dimensional system along the thickness of the specimen, to calculate Tw from dT/dx at each nodal point until the wall temperature Tw = Tc), the value of 'h' and then the Nusselt number 'Nu' can be determined. REFERENCES 1 Marroquin, I. Results of heat transfer tests of the integrated vehicle using phase change paint techniques. INASA Report, No. CR167-695 (1976) 2 Ireland, P. T. and Jones, T. V. The measurement of local heat transfer coefficients in blade cooling geometries. Presented at the AGARD Co~nference, 390, Paper no. 28 (1985) 3 Ireland, P. T. and Jones, T. V. Detailed measurements of heat transfer on and around a pedestal in fully developed passage flow. Presented at the Eiohth International Heat Transfer Conference, San Francisco, USA (1986) 4 Jambunathan K, Edwards, R. J. and Button, B. L. Convective heat transfer coefficients: the colour change paint technique. J. Applied Energy, 2g(2), 137-152 (1987) 5 Edwards, R. J. Enhancement of heat transfer in smooth annular ducts using longitudinal fins or swirling flow. PhD thesis CNAA, Trent Polytechnic, Nottingham (1987). 6 Uzel, A. R. and Button, B. L. Guidelines for expert system applications. Chartered Mechanical Enoineer, 40-45 (February 1987) 7 Intelligent Systems International Ltd, Oakdene Road, Redhill, Surrey, RH1 6BR, UK 8 Private communication with Optimum Heat Control Ltd, Station Works, Dedmere Road, Marlow, Bucks., SL7 1PD, UK 9 Private communication with BDH Ltd, Broom Road, Poole, BH12 4NN, UK 10 Private communication with Thermindex Chemicals and Coatings Ltd, PO Box 191, Mold, Clwyd, CH7 3PS, UK