Transient heat transfer performance of stainless steel structured surfaces combined with air-water spray evaporative cooling at high temperature scenarios. (25th March 2017)

• Record Type: Journal Article
• Authors: Aamir, Muhammad; Qiang, Liao; Hong, Wang; Xun, Zhu; Wang, Jiaqiu; Sajid, Muhammad
• Is Part Of: Applied thermal engineering. Volume 115(2017)
• Page Start: 418
• Page End: 434
• Publication Date: 2017-03-25
• Subjects: Two-phase spray cooling -- Transient heat transfer -- Structured surfaces -- Surface heat flux -- Heat transfer coefficient; Heat engineering -- Periodicals; Heating -- Equipment and supplies -- Periodicals; Periodicals; 621.40205
• DOI: 10.1016/j.applthermaleng.2016.12.126
• Languages: English
• ISSNs: 1359-4311

Abstract

Highlights: All boiling regimes were clearly observed for all experiments performed in initial sample temperature range 600–900 °C. Highest cooling rate of 166.21 °C/sec is achieved with wide square fin structured surface. An enhancement of 47.1, 50.3, 60.1 and 67.1% in burnt out heat for wide square fin structured surface has been achieved. A sharp increase in heat transfer coefficient is observed when cooling process enter into nucleate boiling regime. Boiling Number for pyramid narrow fin and square wide fine is higher than smooth flat surface. Edited Abstract: The objective of the study is to enhance heat transfer performance of stainless steel structured surfaces with different geometries at different initial sample temperature (T s ) under air-water spray having constant spray parameters. Square fin, straight fin and pyramid fin (2 geometries each) are machined on stainless steel blocks. A smooth reference surface (FL) is used as base line. Commercial inverse heat conduction solver INTEMP is used to estimate the time-varying surface heat flux and surface temperature. Burnout heat flux ( qb ) and critical heat flux ( qc ) showed significant increase for wide square fin (Sq-W) sample at all tested sample temperatures with respect to smooth reference surface. The highest cooling rate of 166 °C/sec was achieved with Sq-W sample for Ts = 900 °C. In addition, heat transfer coefficient, h increases gradually with decreasing surface super heat (ΔT). A sharp increase in heat transfer coefficient is observed when cooling process enter into nucleate boiling regime. Wide square fin structured surface showed h-curve sharp increase in nucleate boiling regime earlier than other tested samples. Boiling Number ( Bo ) for pyramid narrow fin (Py-N) and square wide fin (Sq-W) is higher than smooth flat surface (FL). Abstract: The objective of the study is to enhance heat transfer performance of stainless steel structured surfaces with different geometries at different initial sample temperature (T s ) under air-water spray having constant spray parameters. Seven sample structured surfaces has been used. Square wide fin (Sq-W), square narrow fin (Sq-N), straight wide fin (Str-W), straight narrow fin (Str-N), pyramid wide pins (Py-W), pyramid narrow fin (Py-N) were machined on the top surface of the stainless steel cylindrical blocks. A smooth reference surface (FL) was also machined to be used as base line. Top surface diameter of each block was 27 mm and bottom surface diameter was 25 mm. The height, H of each cylinder was 12.5 mm. Each cylinder was providing with two thermocouple holes of 2 mm diameter at different geometrical locations. Square narrow fin (Sq-N) surface, gives maximum enhanced surface area (AE ) of 1689.1 mm 2 with an AE /AS ration of 2.9. Nozzle to surface distance (y) spray angle (θ) and fluid temperature (T f ) are fixed to 25 mm, 0° and 23.5 °C. Commercial inverse heat conduction solver INTEMP was used to estimate the time-varying surface heat flux and surface temperature of the quenched samples. It was determined that geometry of structured surfaces has significant effect on heat transfer rate. Burnout heat flux ( qb ) and critical heat flux ( qc ) showed significant increase for Sq-W at all tested sample temperatures with respect to smooth reference surface. The qc for Py-N is 85.5% and 58.3% higher than Sq-W at Ts = 800 °C and Ts = 900 °C respectively. The highest cooling rate of 166 °C/sec was achieved with sample Sq-W for Ts = 900 °C. In addition, heat transfer coefficient ( h ) increases gradually with decreasing surface super heat (ΔT). A sharp increase in heat transfer coefficient is observed when cooling process enters into nucleate boiling regime. Boiling Number, Bo for FL is smaller than for Py-N and Sq-W which is another way to see better heat performance of these two structured surfaces; making them better choice for high temperature safety applications.