Paper received: 2011-03-17, paper accepted: 2011-11-23 ©2012 Journal of Mechanical Engineering. All rights reserved. Effect of Agitation Work on Heat Transfer during Cooling in Oil ISORAPID 277HM Bohumil Taraba* - Steven Duehring - Jan Španielka - Štefan Hajdu Slovak University of Technology in Bratislava, Faculty of Materials Science and Technology, Institute of Production Systems and Applied Mechanics, Slovak Republic The article focuses on the issue of heat treatment. The cooling curves were obtained for Isorapid 277HM with an experimental way of temperature measuring and their statistical processing. Experimental method was consistent with the test normative ISO standard 9950 (Wolfson's test). The cooling oil Isorapid 277HM was agitated with different agitation work and had a constant temperature of 50 °C. In the next part of this article the surface temperature depended combined heat transfers were calculated. The methodology was based on inverse heat transfer. The interpretation code was software ANSYS and ORIGIN. Keywords: quenching, cooling curve, agitated oil, heat transfer, Wolfson's probe, ANSYS 0 introduction Heat treatment is a multiparameters process. The selection of appropriate parameters predicts required behaviours of treated components. The kind of quenching medium, the selection of quenching medium temperature and the selection of the medium state (unagitated, agitated) are determining factors. Quenching oil Isorapid 277HM belongs to cooling oils common in use. A prediction of treated components behaviour during a cooling process is possible only in the case if the boundary conditions of the process are defined. Before the application of a cooling process numerical simulation, the heat transfer coefficient on the component surface should be defined quantitatively. The experiment, applying simulation model and numerical solution, is able to test the influence of heat treatment parameters on an immediate and final state of a component. A cooling curve is the basis for determining the combined heat transfer coefficient (HTC) as a function of temperature. The current situation presents two ways of getting the HTC cooling curve: direct and inverse approach. Direct access is represented in the publication [1]. HTC is obtained by calculating based on the classical theory of heat conduction in infinite long cylinder with small Biot's number (Bi < 0.1) in few simple recursive computations using the "Heat Transfer Coefficient Wizard". The comparison between measured cooling curves (derived cooling rate curve) with calculated curves is only visual. The heat transfer coefficient inverse method is based on iterative approach loading simulation model in the form of HTC and the effect of temperature at thermocouple (TC-temperature) [2] to [4]. The inverse numerical method is implemented in the software SQintegra also. This program is used as the evaluation tool of the IvfSmartQuench instrument [2]. ! i N Fig. 1. ISO probe cooling process in Isomax 166, a) vapour blanket (VP) in time 1 s, b) begin of boiling (B) in time 2.8 s, c) boiling (B) and convection (C) in time 6.3 s Inverse-numerical-correlation method (INC) defines HTC over inverse heat transfer problem, which was proposed by the authors of this article. The INC method is applied to the solution of direct well-posed inverse problems. Through the controlled iterative process a result which is very likely and useful for computer prediction of thermal treatment processes can be found. The main active part of this procedure is a researcher with theoretical knowledge and experiences with numerical analyses. Typical for the inverse methods is that there exist an infinite set of solutions in general. Only the right setting of statistical criteria get the result with the high degree of reality. Fig. 1 shows the cooling process of ISO probe in optically transparent quenching oil Isomax166. Photographs in Fig. 1 show that the cooling process 102 *Corr. Author's Address: Faculty of Materials Science and Technology, Slovak University of Technology, 91724 Trnava, Slovak Republic, bohumil.taraba@stuba.sk in the three forms of heat transfers (radiation, boiling and convection) are a continuous process without step change (photo from article authors). Then, the determinate HTC must also be continuous. The methodology and results of a cooling effect quantification of oil Isorapid 277 HM with chosen agitation work at temperature 50 °C are presented in the article. 1 experimental Quenching oils ISORAPID are accelerated quenching oils with very good evaporation stability and fast quenching properties. These oils have been especially designed for an application in sealed quench furnaces. They ensure rapid and homogeneous cooling of all parts during batch quenching and also rapid decay of the vapour blanket within the batch. Their application in open quench baths reduces smoke and flame formation significantly [5]. The experimental set-up in Fig. 2 consisted of electrical resistance furnace of LM 212.10 type, cylinder-shaped experimental probe (Table 1, Fig. 3), oil Isorapid 277HM with a mass of 28 kg, portable USB-based DAQ for thermocouples NI USB 9211 for a digital record of measured temperatures, frequency converter MICROMASTER 440 (MM440), a personal computer and pneumatic manipulator for probe moving. A material of probe was austenitic stainless steel DIN 1.4841 with high temperature resistance. Thermophysical material properties were obtained from experimental measuring by NETZSCH apparatus: LFA 427, DSC 404 C Pegasus and Dilatometer 402 C. Geometrical and initial conditions of the experiment were based on the quenching test ISO 9950 [6]. Before cooling, the probe was heated to the initial temperature of 850 °C. The temperatures were measured by the standard 304SS thermocouple of K type with diameter of 1.53 mm located in the centre of the probe. Temperatures were recorded 5 times per second. A set of measurements was repeated six times for each state of oil. Each set of measured cooling curves was averaged into a core cooling curve. There were seven oil states realized, one for unagitated and six for agitated. Temperature measurement started from the moment when the centre of gravity of probe reached the oil level. Power parameters (torque moment and input rpm) of the swirl devices were obtained from the data of frequency converter MM440. Table 1. Thermophysical material properties of austenitic stainless steel DIN 1.4841 T [°C] A [W-m-1-K-1] p [kgm-3] cp [Jkg-1K-1] 0 13.5 7880 474 100 15.0 7854 490 200 16.8 7814 512 300 18.6 7773 525 400 20.0 7731 535 500 21.3 7689 544 600 23.2 7645 569 700 24.8 7601 581 800 25.6 7556 589 900 27.1 7511 600 Fig. 2. Experimental setup: 1- electrical resis-tant furnace, 2- personal computer, 3 - probe with a thermocouple, 4 - NI USB 9211 converter, 5 - cooling medium and its heater, 6 - pneumatic manipulator, 7- record of cooling curve, 8- frequency inverter 2 theoretical base of the task Transient temperature field T = T(r, z, t) of a cooled probe is described by Fourier-Kirchhoff differential equation (FKDE) of heat conduction for cylindrical coordinate system [8], MT ) dT _ ~di _ p(T)cp (T) d 2t 1 dT + d 2T dr2 r dr dz2 [K s-1], (1) where X(T) is coefficient of heat conductivity, p(T) density, cp(T) specific heat, r radius [m] and z is height of probe [m]. Combined heat transfer coefficient HTC(Ts) was determined as the function of the probe surface temperature Ts for constant oil temperature Tr. The condition of equality of heat flux is valid on the probe surface in time point ti by formula [4]: -X(T) gradT\ti = HTC(Ts) [Ts ()-Tr ] . (2) Other assumptions of thermal tasks: the probe material is isotropic and its thermophysical properties are temperature dependent, the cooling process is isobaric, the temperature field is not dependent on the angle p, T f fp), coolant temperature is constant throughout the process, Tr f f(t). Heat generation in unit volume per unit time was not take account because in probe material there are no phase transformations in the temperature interval 50 to 850 °C. A thermal task is solved by the finite element method (FEM). The FEM solution procedure is in the form of equation: K1 ■ T + K 2 ■ T + K3 ■ T - P = 0, (3) where K is heat conduction matrix, K2 matrix of boundary conditions, K3 enthalpy matrix, T temperature vector, T time derivation of temperature and P is vector of outer loads. Absolute value of relative error dT was obtained by formula: §J = T - T TC ans T ± 7V 100 (4) where Ttc is measured temperature and Tans is temperature of numerical solution, both for time tt. Input power into oil per 1 kg Pw was calculated from torque moment and angular velocity values at device for swirling by formula: P = 2n M n (5) m where MT is torque moment, n rotational speed and m is mass of oil in device. 3 numerical simulation Engineering-scientific program code ANSYS [7] was the interpretation program of numerical simulation. Geometrical model of the probe was the lower half part of the cylinder, Fig. 3. Applied elements were axisymmetric with linear base function and surface temperature behaviour option. Surface temperature behaviour allows the application of thermal load HTC(Ts) as the actual surface temperature function. The generated mesh was mapped with the length of the element edge 0.25 mm. Calculation procedure was transient and nonlinear. Time step was 0.01 s. Fig. 3. Probe geometry and geometrical model with generated mesh Fig. 4. Block diagram of iterative solution of the boundary condition - INC method Through the solution of simulation model of thermal nonlinear and transient task in the ANSYS the temperature curve for chosen HTC-loads values was found. Then, a comparison with measurement temperature curve followed and the process was repeated. The curve fitting takes account of the temperature and cooling rate curve. Task solution by the INC method must meet the following criteria: absolute value of relative error for measured and calculated temperature in the /-time must be less than 1%, absolute value of relative error for cooling rates derived of measured and calculated temperature must be less than 5% and the correlation coefficient between the measured and calculated temperatures in the cooling time must be greater than 0.99. Block diagram of iterative solution of the boundary condition - INC method is showed in Fig. 4. 4 obtained results Time dependences of 7 measured temperatures during probe cooling from 850 °C into unagitated and agitated oil at temperature 50 °C are shown in Fig. 5. These core cooling curves were the basis for INC method applying. Fig. 5. Set of measured temperatures, unagitated and agitated oil ISORAPID 277HM In Fig. 6 are plotted cooling rates curves (derived from core cooling curves). There is a distinct difference between cooling in unagitated and agitated oil. The lowest value of cooling rate is for unagitated oil and with energy supplied into oil increases the cooling rate and temperature at the centre at which the maximum cooling rate. The cooling rate interval is of 103 to 114 Ks-1. Combined heat transfer coefficient dependences of probe surface temperatures for unagitated and agitated oil are the main results of INC method and are shown in Fig. 7. 4500 y 4000- "e g 3500-G ¡j 300025002000150010005000 ISORAPID 277HM, 7=50 °C pw[js"1k1] r— Pw=4.80 /ft Pw=2.5a \//f 1 Pw=1.65, n Pw=0.78 )