Effect of Internal Structure of Nozzle on Impingement Heat Transfer Performance of Single-beam Water Jet

Abstract

Ultra-fast cooling (UFC) equipment is an important means to produce high-quality hot-rolled steel materials. Nozzle is the key component of the UFC equipment, which has great influence on the cooling performance. In this paper, the influence of internal structure of the nozzle on the heat transfer performance of water jet impingement was studied experimentally, so as to obtain an excellent nozzle structure. Under the same inlet pressure and outlet flow of the nozzle, heat transfer experiments were carried out on a 750°C stainless steel plate. During the experiment, the actual flows of the nozzles and the temperatures inside the steel plate were recorded. Flow coefficients of the nozzles, surface temperatures and surface heat flux of the steel plate were calculated. Experimental results showed that the flow coefficient of the 30° nozzle was the largest, followed by the 13.4° nozzle, and then the 45° nozzle. It could be found that, under the same inlet pressure, heat transfer performance was positively related to the flow coefficient of the nozzle. Moreover, under the same outlet flow, the heat transfer performance of each nozzle was very similar. Overall, the heat transfer performance of the 30° nozzle was excellent and recommended to be used preferentially in the UFC equipment.

1. Introduction

With the development of industrialization, the demand for high-performance steel materials is increasing in engineering machinery, marine engineering and other fields. As an important means to improve the performance of hot-rolled steel materials, UFC technology is widely used increasingly.^1,2,3,4,5) It uses water jet to cool the rolled steel on the hot rolling line quickly and uniformly. It can reduce the amount of alloy elements, simplify the production process, save energy, help enterprises to reduce production costs and improve product performance.^6,7,8,9,10)

Increasing the cooling capacity can improve the production efficiency and provide powerful means for developing new types of steel. Lots of research had been done in this field. Merci¹¹⁾ et al. studied the effects of jet impingement height and Reynolds number on heat transfer performance. Chester and Hauksson et al.^12,13) found that increasing flow rate could enhance the heat transfer ability of the hot plate surface. Öztekin et al.¹⁴⁾ found that the increase of the roughness of the material surface will enhance the heat transfer ability. Ai et al.¹⁵⁾ found that the use of mobile nozzles instead of fixed nozzles could improve the scour ability and enhance heat transfer. Glaspell et al.¹⁶⁾ found that reducing the distance from the nozzle to the target surface could improve the heat transfer performance at the stagnation zone. Lv et al.¹⁷⁾ found that the heat transfer coefficient of Al₂O₃-water nanofluid jet was 61.4% higher than that of pure water under the same Reynolds number. Sorour et al.¹⁸⁾ and Barewar et al.¹⁹⁾ found that the increase of the volume fraction of SiO₂ and ZnO nanofluids was beneficial to the enhancement of the heat transfer performance. Sorour et al. found that the average Nusselt number increased significantly on the impact surface when the volume fraction of SiO₂ nanoparticles increased from 0% to 8.5%. And when the volume fraction of ZnO nanofluids increased from 0.02% to 0.1%, Barewar et al. found that the cooling velocity of the stagnation zone increased by 54.7%. Nobari et al.²⁰⁾ found that increasing nozzle flow could increase the heat flux of the whole steel plate surface when the cooling water temperature was constant. Moreover, they found that when the flow rate was constant, reducing the cooling water temperature could also enhance the heat transfer effect of the steel plate surface. Ghasemian et al.²¹⁾ compared the influence of water, Newtonian molten salt and non-Newtonian molten salt on the heat transfer performance of steel plate in the process of free jet impingement. It was found that the cooling effect of non-Newtonian molten salt was the strongest, followed by Newtonian molten salt, and then by water.

Nozzle is the key component of the UFC equipment. Optimizing its structure is essential for improving the cooling performance of the equipment. However, the effects of nozzle structure on jet impingement heat transfer performance were not involved in the above studies. Zhang et al.²²⁾ simulated the heat transfer laws of cylindrical nozzle and straight cone nozzle with 30° cone angle and found that the heat transfer performance of the straight cone nozzle was better than that of the cylindrical nozzle. Wen et al.²³⁾ carried out jet impingement experiments on three straight cone nozzles with cone angles of 60°, 90° and 120° respectively, and found that 60° cone angle was beneficial to the improvement of the jet impingement ability. Zhang et al.²⁴⁾ found that a straight cone nozzle with 13°cone angle had better jet impingement performance than a conical nozzle. Through a spray cooling experiment of metal plate, Bellerová et al.²⁵⁾ and Tseng et al.²⁶⁾ found that the conical nozzle could make the entire circular impingement area have a very uniform distribution of liquid flow.

However, there is no clear conclusion on how to choose cone angle to maximize the flow coefficient of the straight cone nozzle. In addition, there have been no reports on experimental research using a water jet, produced by a straight cone nozzle, to cool high-temperature steel plate. This paper intends to carry out some work in this area. It is hoped that the research results will contribute to the development and application of ultra-fast cooling equipment.

2. Principle of Jet Impingement Heat Transfer

Free jet impingement cooling is one of the important cooling methods in ultra-fast cooling technology. The left half of Fig. 1 shows the flow process of cooling water during free jet impingement. When cooling water jet impacts on the plate surface, a stagnation zone is formed at first. It is a transition zone where high-pressure water flows from axial to radial direction along the plate surface. The cooling water develops in the radial direction to form wall jet zone. Part of kinetic energy is converted into potential energy, which makes liquid level rise and produces hydraulic jump. The continuous impact of cooling water makes wall jet zone expand gradually, and pushes the hydraulic jump to move along radial direction until the whole surface is wetted. The right half of Fig. 1 shows the heat transfer mechanism between cooling water and hot surface during jet impingement. When cooling water impinged onto plate surface, it could go through the following fourth stages: bubble generation and growth, steam film formation, steam film rupture, and bubble separation. Stagnation zone has maximum static pressure, bubbles produced by boiling are broken before they grow up, and so stable vapor films cannot be formed. Outside the stagnation zone, the farther away from the stagnation zone is, the stronger the bubble growth ability is, and the easier it is to form stable steam films. The ability to sweep vapor film largely depends on radial flow ability of the cooling water.

Fig. 1.

Schematic diagram of jet impingement process.

3. Experimental Materials and Methods

3.1. Structure of the Experimental Nozzles

The basic requirements for the nozzle used in UFC equipment are strong flow capacity (having large flow coefficient) and excellent bunching performance (no divergence of water flow). Although cylindrical nozzle has excellent bunching property, its flow coefficient is small, only about 0.8. Conical nozzle has larger flow coefficient, but its bunching property is poor. At present, the straight cone nozzle shown in Fig. 2 is widely used in UFC equipment. It can be divided into three parts: the inlet section (I), the contraction section (II), and the outlet section (III). The inlet section is usually connected to a high-pressure pipeline. Straight cone nozzle is a high efficiency nozzle with an excellent jet performance. The basic characteristics of the straight cone nozzle include a cone angle of θ, an inlet diameter to outlet diameter ratio of D/d=2–3 and a cylinder length to outlet diameter ratio of l/d=2–4.

Fig. 2.

Schematic diagram of straight cone nozzle structure.

In some ways, its structure is quite similar to the conical nozzle. Their main difference is the structure of the outlet section. The conical nozzle has no outlet section. Compared with conical nozzle, straight cone nozzle has higher flow coefficient and better jet density.²⁷⁾ Large amount of experimental data show that the flow coefficient of the conical nozzle can reach the maximum when the cone angle is 13.4°. And 13.4° is also known as the optimum cone angle of the conical nozzle.²⁸⁾ According to the results of reference [22], [23] and [24], 13.4°, 30° and 45° were taken as the cone angles of the experimental nozzles, respectively. The other parameters were the same as those in an existing UFC equipment, that was, the diameter of the cylinder at the inlet of the nozzle was D=6.5 mm, the diameter of the cylinder at the outlet was d=3 mm, the length of the cylinder at the outlet was l=6 mm, and the total length of the nozzle was L=25 mm.

3.2. Experimental Steel Plate

The experimental steel plate was made of AISI 304L stainless steel with a size of 20 mm×80 mm×150 mm. As shown in Fig. 3, five holes of 3 mm in diameter and 30 mm in depth were drilled at the position of 2.5 mm beneath the plate surface, and the spacing between these holes was 10 mm. A K-type armored thermocouple, whose measuring temperature range is 0–1100°C, was inserted in each hole. The narrow gaps between the thermocouples and the holes were filled with high temperature thermal conductive sealant. For the convenience of experiment, mark the stagnation point on the upper surface of the steel plate.

Fig. 3.

Steel and thermocouples layout (Point 1 is the jet impingement point).

3.3. Experimental Facilities

As shown in Fig. 4, the experimental facilities consisted of heating, water supply, data recording and other systems or devices. A vortex pump was used as the power source of the water supply system, whose maximum water supply pressure was 1.2 MPa. A crawler heating belt, whose maximum heating temperature was 1050°C, was used to heat the experimental plate. The temperature of the water was maintained at 20°C and the distance from the nozzle outlet to the steel plate was H=200 mm.

Fig. 4.

Experimental device schematic.

Real-time temperatures of the five measuring points were recorded and saved by GRAPHTEC GL220 data collector every 0.1 seconds. Based on the interior temperatures collected and heat differential equation, temperature fields and heat fluxes on the plate upper surface were calculated with inverse heat conduction method. Considering the change of thermophysical parameters with temperature, established a two-dimensional unsteady heat conduction equation along the thickness and width of the steel plate. The initial conditions and boundary conditions of the heat conduction equation on the upper surface of the steel plate were set. The completely implicit Crank-Nicolson difference decomposition method was used to alternately solve the temperature in the thickness and width directions of the steel plate. The temperature T_W and heat flux Q_W on the plate surface can be calculated when substituting the temperature recorded in the data collector into the heat conduction equation.^{29,30,31,32,33)}

The uncertainties of the measured data were as follows: the volume flows were ±0.05 L/min, the inlet pressures were ±0.02 MPa, the diameters of the outlet section were ±0.01 mm, the water temperature was ±0.5°C, and the plate temperatures were ±1°C. In addition, the position deviations of the thermocouples were ±0.2 mm, the deviations of the inlet section diameter, the total length and the outlet section length of the nozzles were 0.1 mm, and the deviations of the cone angles were 0.05°.

3.4. Experimental Methods

Experiment 1: Heat transfer experiments under the same inlet pressure. The inlet pressure of the nozzle was set to 0.2 MPa, 0.4 MPa and 0.6 MPa, respectively. The experiment processes were as follows. Step 1: Heat the plate to 770°C and keep it for half an hour. Step 2: Turn on the pump and adjust pressure to the target value. Step3: Close the shut-off valve. Step 4: Fix the plate on the test table and connect thermocouples with the data collector. Step 5: Open the shut-off valve when the plate temperature monitored by the data collector drops to 750°C. Step 6: Collect the unstable water flow with a container after opening the shut-off valve and remove the container when the water pressure stabilize to the target value, and then the heat transfer experiment started. Repeated the experiment several times to reduce experimental errors.

Experiment 2: Heat transfer experiments under the same outlet flow. Keep the above average outlet flow of the 30° nozzle unchanged, fine tune the inlet pressure of the other two nozzles to make the outlet flow consistent with the 30° nozzle. Other steps were the same as those in Experiment 1.

During the experiment, the water jets were in perfect cylindrical shape, and no broken droplets were found.

4. Analysis of the Experimental Results

4.1. Flow Coefficient of Nozzle

Flow coefficient indicates the maximum flow capacity of the nozzle. The larger the flow coefficient is, the smaller the pressure loss of the flow through the nozzle is. Flow coefficient was calculated according to Eq. (1):   

$${\text{C}}_{\text{d}}=\frac{\text{Q}}{\text{A}}\sqrt{\frac{\rho }{\text{2}\mathrm{\Delta }\text{P}}}$$(1)

Where Q is the volume flow through the nozzle, m³/s; A is the cross-sectional area at the nozzle outlet, m²; ρ is the fluid density, kg/m³; ΔP is the pressure drop when the fluid passes through the nozzle, Pa.

The flow coefficients of the three nozzles calculated from the flow data measured in Experiment 1 are shown in Table 1. Table 1 also shows the error of each parameter.

Table 1. Flow coefficient of two nozzles.

Cone angle	Inlet pressure (MPa)	Volume flow (L∙min−1)	Flow coefficient
13.4°	0.2	7.93	0.935
13.4°	0.4	11.29	0.941
13.4°	0.6	13.78	0.938
30°	0.2	8.23	0.970
30°	0.4	11.67	0.973
30°	0.6	14.23	0.969
45°	0.2	7.54	0.889
45°	0.4	10.73	0.894
45°	0.6	13.09	0.891

According to Table 1, the average flow coefficients of 13.4°, 30° and 45° nozzles are 0.938, 0.971 and 0.891, respectively. It can be seen that the flow coefficient of 30° nozzle is the largest, followed by 13.4° nozzle.

4.2. Comparison of Heat Transfer Performance under the Same Inlet Pressures

Since multiple sets of experimental data can not be displayed one by one, an experiment is selected to show the change curve of temperature and heat flux with time during the experiment. As shown in Fig. 5, when the impact pressure is 0.2 MPa and the nozzle cone angle is 30°, the temperature and heat flux change curve at each position on the plate surface. It can be found that the closer to the stagnation point, the faster the temperature begins to drop, and there is a larger heat flux peak (MHF value) after the jet impact.

Fig. 5.

Variation curve of temperature and heat flux with time during the experiment. (a) Surface temperature change curve with time (b) Surface heat flux change curve with time. (Online version in color.)

When the inlet pressure of the nozzles was 0.2 MPa and 0.6 MPa, the maximum heat flux (MHF) values and the times to reach the MHF values at the stagnation point, 10 mm, 20 mm, 30 mm, 40 mm and 50 mm away from the stagnation point on the plate surface are shown in Figs. 6 and 7. It can be seen that, with the increase of the distance from the stagnation point, the MHF value decreased and the time to reach the MHF value increased on the plate surface. Meanwhile, the closer the distance to the stagnation point was, the more obvious the influence of the inlet pressure on the improvement of heat transfer performance was. When inlet pressure increased from 0.2 MPa to 0.6 MPa, the MHF values at the stagnation point of 13.4°, 30° and 45° nozzles increased 21.8%, 23.2% and 20.4%, respectively. While the values at 20 mm away from the stagnation point only increased 11.8%, 9.4% and 13.2%. It can also be found that increasing the inlet pressure can also shorten the time to reach the MHF value on the whole plate surface.

Fig. 6.

Comparison of heat flux at the jet impingement point under the same inlet pressure. (Online version in color.)

Fig. 7.

Comparison of time to MHF values at the same inlet pressure. (Online version in color.)

From Fig. 6, it was clearly observed that the heat transfer performance of the 30° nozzle was the best, and that of 45° nozzle was the worst. It can also be seen that the heat transfer performance was positively related to the flow coefficient of the nozzle under the same inlet pressure. When the pressure was 0.2 MPa, the MHF value of 30° nozzle at the stagnation point was 0.09 MW∙m⁻² higher than that of 13.4° nozzle and 0.31 MW∙m⁻² higher than that of 45° nozzle. In brief, the MHF value of 30° nozzle was the largest and the time to reach the MHF value was the shortest.

The influence of the inlet pressure on the average cooling rate, within 3 seconds after jet starts at the stagnation point and 10 mm away from the stagnation point, is shown in Fig. 8. It is clearly shown that the cooling rate increased with the increase of the inlet pressure of the nozzle, the cooling rate at the stagnation point was the largest, and the cooling rate of the 30° nozzle was the largest.

Fig. 8.

Effect of nozzle inlet pressure on average cooling rate. (Online version in color.)

According to Eq. (1), when the cross-sectional area of the outlet is constant, the flow through the nozzle is directly proportional to the flow coefficient and the square root of the pressure drop. Since the outlet pressure is constant (approximately 1 standard atmospheric pressure), the pressure drop is equal to the inlet pressure. The increase of inlet pressure or flow coefficient results in the increase of volume flow through nozzle. It can be concluded that the outlet volume flow is the main factor affecting the cooling rate when the cross-sectional area of the nozzle outlet was constant. The greater the volume flow is, the greater the cooling velocity is. For example, the flow coefficient of 30° nozzle was about 9% higher than that of 45° nozzle when inlet pressures were 0.6 MPa, resulting in the cooling rates of 30° nozzle were 4.7% and 17.2% higher than those of 45° nozzle at the stagnation point and 10 mm away from the stagnation point.

Figure 9 shows the relationships between heat flux and temperature of 13.4° nozzle and 30° nozzle on the surface of steel plate at the position of 0 mm, 10 mm and 20 mm away from the stagnation point under the inlet pressure of 0.2 MPa. At the positions of 0 mm and 10 mm, influenced by jet, bubbles separated from plate surface rapidly, and could not form stable vapor films, so there was no obvious film boiling process. With the decrease of the surface temperature, heat flux gradually increased to MHF value, and then directly entered into the nucleate boiling stage. After that, with the continuous decrease of the surface temperature, heat flux also decreased. While, at the position 20 mm, when heat flux increased to MHF value, it showed an approximate horizontal segment AB. Where, heat flux was almost unchanged as temperature continued to decline. AB segment was the temperature range on the plate surface when stable film boiling occurred. It showed that film boiling process lasted longer at the place far away from the stagnation point. When the bubble diameter reached the critical diameter of rupture, heat transfer mode changed from film boiling to nuclear boiling.

Fig. 9.

Boiling curves at different positions on the plate surface under 0.2 MPa pressure. (Online version in color.)

At the place 0 mm, 10 mm and 20 mm away from the stagnation point, the temperatures of the 13.4° nozzle reaching the MHF value were 522.4°C, 494.1°C and 452.3°C, respectively. While, those of the 30° nozzle were relatively high, which were 549.8°C, 512.9°C and 481.5°C, respectively. Compared with the 13.4° nozzle jet, the 30° nozzle jet reached a higher MHF value at a higher temperature on the whole steel plate surface, and entered into the nucleate boiling stage earlier, resulting in a higher convective heat transfer capacity. It can be concluded that under the same inlet pressure, the larger the flow coefficient of the nozzle is, the shorter the film boiling stage is, and the earlier it is to enter the nucleate boiling stage with higher heat transfer capacity.

4.3. Comparison of Heat Transfer Performance under the Same Outlet Flows

Since the flow coefficient of the 30° nozzle was the largest, in order to make the outlet volume flows of the three nozzles the same, the inlet pressures of the other two nozzles were increased appropriately, as shown in Table 2.

Table 2. Comparison of heat flux at the jet impingement point at the same outlet flow rate.

Flow rate (L∙min−1)	Inlet pressure (MPa)
	30° nozzle	13.4° nozzle	45° nozzle
8.23	0.2	0.21	0.24
11.67	0.4	0.43	0.48
14.23	0.6	0.67	0.71

When the volume flow was 8.23 L∙min⁻¹ and 14.23 L∙min⁻¹, the MHF values and the times to reach the MHF values, at the points 0 mm, 10 mm, 20 mm, 30 mm, 40 mm and 50 mm away from the stagnation point on the plate surface, of the three nozzles were shown in Figs. 10 and 11. For a fixed flow rate, the characteristics of MHF in Figs. 10 and 11 are the same. The MHF values increased significantly when the volume flow increased from 8.23 L∙min⁻¹ to 14.23 L∙min⁻¹. It is observed that increasing volume flow is helpful to the improvement of heat transfer performance. Under the same outlet volume flow, the heat transfer performance of the 30° nozzle was slightly better than that of the other two nozzles, but the MHF value of each nozzle at the same point was very similar, the maximum difference was only 0.06 MW∙m⁻². Considering the influence of the experiment error, we can conclude that the heat transfer characteristics are only determined by the nozzle flow rate, and the cone angle has no effect on the cooling rate and MHF. However, the 30° nozzle of the three types of nozzles has the largest flow coefficient, indicating that the pressure provided by the pump is the smallest for the same flow. Since the driving power of the water pump is proportional to the discharge pressure and flow rate, when the flow rate is fixed, the input power is the smallest when the 30° nozzle is used. This is beneficial to the energy saving of UFC equipment. To achieve the same heat transfer capacity, lower inlet pressure could be used when the 30° nozzle was selected, which was beneficial to energy saving of the UFC equipment.

Fig. 10.

Comparison of MHF under the same outlet flow rate. (Online version in color.)

Fig. 11.

Comparison of time to MHF at the same outlet flow rate. (Online version in color.)

Tables 3 and 4 show the corresponding data of the outlet flows and the average cooling rates within 3 seconds after jet started at 0 mm and 10 mm away from the stagnation point when the three nozzles jet impinged on the steel plate. From these data, we can also draw the same conclusion as from Figs. 9 and 10, which would not be repeated here.

Table 3. Comparison of average cooling rates at 0 mm from stagnation point under the same outlet flows.

Flow rate (L∙min−1)	Cooling velocity at 0 mm from stagnation point (°C∙s−1)
	13.4° nozzle	30° nozzle	45° nozzle
8.23	181.6	183.6	171.2
11.67	197.1	198.7	189.7
14.23	213.2	213.9	207.3

Table 4. Comparison of average cooling rates at 10 mm from stagnation point under the same outlet flows.

Flow rate (L∙min−1)	Cooling velocity at 10 mm from stagnation point (°C∙s−1)
	13.4° nozzle	30° nozzle	45° nozzle
8.23	130.6	137.8	122.1
11.67	147.7	154.7	141.5
14.23	174.8	175.2	172.2

5. Conclusions

(1) The flow coefficient of the straight cone nozzle with 30° cone angle was 0.971, which was the largest among the three nozzles studied and had the best flow capacity.

(2) Under the same inlet pressure, 30° nozzle jet could reach a higher MHF (maximal heat flux) at a higher temperature on the whole steel plate surface, and its heat transfer capacity was the strongest.

(3) Under the same outlet flow, the heat transfer capacity of 30° nozzle was slightly better than that of the other two. Lower pressure is required if 30° nozzle is adopted, which is beneficial to energy saving of the equipment.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (51804074).

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