MAXIMUM HEAT FLUX AND WETTING DELAY DURING QUENCHING OF HIGH TEMPERATURE

Effects of experimental parameters on peak heat flux and wetting delay were also observed in this study. The peak heat flux and wetting delay characteristics are strongly influenced by jet velocity and liquid jet subcooling. Both the maximum heat flux and the wetting delay were found to be almost independent of the initial block temperature.

NOMENCLATURE

CHAPTERl

INTRODUCTION

Jet Impingement Quenching
Boiling Phenomena
Maximum Heat flux during Quenching
Wetting Delay during Quenching
Objectives of this study

The maximum heat flux is estimated from the solid side which cannot be measured directly. Ueda and Inoue [8] found that the maximum heat flux shows a somewhat higher value than the critical heat flux. The thermal properties of the solid material have a significant influence on the maximum transient heat flux during quenching.

Fig. 1.2: Typical pool boiling curve for water at one atmosphere

LITERATURE SURVEY

Many researchers have focused on investigating the complex phcnomcna of the wetting delay in jet impingement quenching. They observed the propagation of the abrupt cooling front in the flow direction along the surface. The jet velocity and jet subcooling were varied for each initial temperature of the steel block as shown in Table 3.3.

Table 3.3 Test conditions of the experiment

DATA ANALYSIS

Material properties for initial temperatures 250-400°C are tabulated in Appendix C. Surface Temperature, Surface Heat Flux Tw, Maximum Surface Heat Flux Qw, Qmax. with time and radial position). Then the output data files of surface temperature and surface heat flux from the inverse solution are analyzed to obtain heat transfer characteristics during quenching of carbon steel materials. Although the experimental investigations of this work have already been carried out by Mozumder [16], the present research has also analyzed the effects of all experimental parameters that have an influence on the wetting delay and peak heat flux phenomena.

Table 4.1 Input temperature data to the inverse solution program TS4002003.txt 0

CHAPTERS

WETTING DELAY AND MAXIMUM HEAT FLUX

Regimes of Wetting Delay

17] examined the residence times for each condition of experiment and divided the residence times into three regimes: (a) rapid cooling, (b) moderate cooling, and (c) slow cooling. He observed that the residence times for steel were very short while the residence times for copper were relatively long. In this case of steel material, it took only a fraction of a second for the wetting front to start moving and the surface tcmpcraturc cooled vcry rapidly immediately after the jet hit the surface.

For copper, the thermal conductivity is high, so heat can easily be supplied to balance the heat flux required by the beam and maintain a high surface temperature. The spatial temperature gradient at the time of residence and during the propagation of the wetting front is also different for three regimes. For shorter residence time regimes, the radial temperature gradient is quite large at and after the residence time.

The temperature gradient in the radial direction is small and the radial propagation of the wetting front is faster for larger residence times. Conversely, it is observed that a higher temperature gradient in the radial direction corresponds to a slower wetting front speed for fast cooling conditions. It is also found that the time period between the onset of movement of the wetting front to the state of maximum heat flux is generally longer in the moderate cooling regime than the slow cooling regimes.

Importance of Maximum Heat Flux

Surface temperature and surface heat flux distribution with time

The surface temperature and surface heat flux estimated from the inverse solution with radial position are shown in Figure 5.4. The heat flux was estimated by the inverse heat conduction method, which represented that the heat flux value also varied with the radial position. The maximum heat flux point on the boiling curve is the boundary between the core boiling region and the transition boiling region.

In the single-phase convection region, the surface temperature is close to the saturation point and the heat flux is small. In the dry region, the surface temperature rises suddenly from 250°C to the initial block temperature of 400°C and the heat flux becomes very small. In order to understand the complex phenomena of the quenching process, it is necessary to obtain the distribution of surface temperature and surface heat flux with time after the impact of the aircraft.

The surface temperature and surface heat flux distribution with time is shown in Fig. The surface heat flux also reaches its maximum value within fractions of a second and then decreases. After a few seconds the change in surface heat flux with time is very low and the heat flux value is very small.

The rapid drop in peak heat flux may be a direct result of the sudden drop in surface temperature.

Fig. 5.4 Regimes of boiling and maximum heat flux during wetting front propagation (T b =400°C, LlT sub =50K, u=3m/s, t=4.3 s) [18]

RESULTS AND DISCUSSIONS

Effect of initial block temperature on maximum heat flux

Mozumder [16] found that the effect of the initial block temperature on the maximum heat flux is very small. The largest differences were observed at the smaller radial position, where the heat flux was slightly higher at a higher initial temperature. The time required to reach the maximum point of heat flow is longer at a higher initial temperature.

Due to its very small effect, the maximum heat flux can be assumed to be almost independent of Tb. Fig. In this case there is very little effect of Tb on the maximum heat flux. In general, it is clear that the block material greatly influences the rate of heat transfer.

Mozumder [16] experimented with beam impingement quenching of three different block materials (brass, copper and steel) and reported that the maximum heat flux for copper material is more than twice that of steel material. The heat from the quench area is quickly recovered by the stored heat from the nearer surface due to its higher thermal conductivity. For steel material, the surface is locally cooled close to the surface on which the beam struck.

Due to the low thermal conductivity, the heat is dissipated locally from the heated surface, but the overall cooling effect is not immediately felt on the entire solid body.

Correlation for maximum heat flux

The experimental data and the proposed correlation for the copper and brass block were found to be in good agreement as shown in the figure. Peak heat flux correlations were developed for two different radial position regions of 10-20 mm and 20 mm. -35 mm in this analysis. Therefore, the maximum heat flux for radial positions is 17s of 10 mm h~s pot beep incl\lqeej hefe.

The lower liquid temperature contributes to the higher sensible heat required for the liquid to reach its saturation state, indicating the higher heat transport capacity to remove the heat in quenching. The term (I+Ja) is included in the correlation and this term has a significant effect for region I. For region II, (I+Ja) is found to be negligible, because the maximum heat flux tends to decrease as the wetting front moves in radial direction.

The coefficient of these two correlations was determined using the least squares method from the experimental data of the maximum heat flow. The experimental data are then compared with these proposed correlations for both regions as shown in the figure. These correlations are not well predicted for initial block temperatures of 450-600 °C as shown in the figure.

A new heated block was placed for the same experiment and the experiment was conducted for the initial temperatures of 450-600°C with this set-up.

Fig. 6.3.6.1 Comparison of qrnaxdata for copper and brass with the proposed correlation (for region II(Il-25 mm» [18]

CONCLUSIONS AND RECOMMENDATIONS

Due to the low thermal conductivity of steel, the liquid stream removed the heat locally from the hot surface, as a result the entire body did not immediately feel the quenching effect. In the present study the wetting delay for most of the experimental conditions could not be determined. More extensive analysis may be needed to find the reasons for such behaviors of the carbon steel block.

Due to the low thermal conductivity of steel, the entire block did not immediately feel the effect of cooling due to the impact of water jets. Therefore, research into effective coolants may be necessary for quenching steel materials or products with lower thermal conductivity.

15] Hammad, 1., "Heat Transfer Characteristics and Wet Front During High Temperature Surface Quenching by Jet Impingement", PhD Thesis, Graduate School of Science and Engineering, Saga University, Japan, 2004. K ., "Thermal and hydrodynamic characteristics of Jet Impingement Quenching for high temperature surface", Ph.D Thesis, Graduate School of Science and Engineering, Saga University, Japan, 2006. A, "Maximum heat flux in relation to quenching of a high temperature surface with jet fluid impingement", Int. .

25] Dutta, Joydip and Hossainy, G.A.,"Investigation of residence time and maximum heat flux for carbon steel during jet impingement quenching" Bachelor Thesis, Dept.

APPENDIX A

Two auxiliary heaters were used to insulate the block and to maintain a uniform heat flux at the surfaces; one of them is placed around the block perimeter with a capacity of 0.65kW and another one placed on the upper part of the block with a capacity of 0.5kW. The thermocouples were scanned sequentially at 0.05 second intervals, with 8.0 ms required to read all the thermocouples using 16-bit resolution with an analog-to-digital converter. The duration of the total data acquisition period was adjusted to match the experimental conditions so that in all cases the experiment continued until quenching was complete.

The uncertainty in the temperature measurements was 0100.46°C, while the uncertainty in the location of the thermocouples was 0100.1mm. A shutter was fitted in front of the nozzle to prevent water from hitting the block prematurely and to maintain a constant water temperature by forcing it to run in a closed loop system. The desired temperature of the water was achieved by controlling the main heater, the auxiliary heater or by adding cooling water to the cooler.

The block was heated to the desired initial temperature by heating it with an electric heater mounted around the block. A dynamic strain gauge was attached at two points of the flow line in front of the nozzle for the measurement of differential pressure from which jet velocity was calculated. The whole experiment was done in a nitrogen atmosphere to prevent oxidation of the test surface.

After all desired initial conditions for the experiment were set, the shutter was opened to allow the water jet to hit the center of the flat surface of the heated surface.