Investigation of the performance of pulsating heat pipe subject to uniform/alternating tube diameters Chih-Yung Tseng a , Kai-Shing Yang a , Kuo-Hsiang Chien a , Ming-Shan Jeng a , Chi-Chuan Wang b,⇑ a Green Energy & Environment Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan b Department of Mechanical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan article info Article history: Received 31 October 2013 Received in revised form 28 January 2014 Accepted 28 January 2014 Available online 5 February 2014 Keywords: Pulsating heat pipe Horizontal arrangement Alternating diameter abstract The present study examines the performance of closed-loop pulsating heat pipes (CLPHPs) with an ID of 2.4 mm. The effect of uniform and alternating tube diameter on the performance is investigated. The working fluids include distilled water, methanol and HFE-7100. Tests are performed with both horizontal and vertical arrangement. For the horizontal arrangement, when compared to uniform design, the alternating channel design can be started at a rather low heat input with a much smaller thermal resis- tance. Normally the thermal resistance is decreased with the rise of heat input, and reveals a minimum value at a certain heat input followed by shows a marginal rise when the heat input is increased further. Both uniform and alternating design reveals the similar trend. For the vertical arrangement, the thermal resistance is much lower than that in horizontal arrangement. Different from that in horizontal arrange- ment, the thermal resistance shows a continuous decline against heat input for all the working fluids. For a low input power, CLPHP with HFE-7100 shows the least thermal resistance. By contrast, CLPHP with distilled water shows the smallest thermal resistance when the input power is increased over 60 W. Ó 2014 Elsevier Inc. All rights reserved. 1. Introduction The number of transistors on integrated circuits followed the so called Moore’s law in which the integrated circuits double approx- imately every two years in a specific area. This eventually leads to an enormous rise of heat density, placing a major roadblock for further miniature. As a consequence, improving the performance of the thermal module is imperative to maintain the junction tem- perature of the electronic devices below certain threshold. One of the most successful heat removal designs is heat pipe which is now used in every aspect of electronic cooling applications. How- ever, convectional heat pipes normally suffers from comparatively lower maximum heat dissipation and shorter operational distance as used in electronic cooling. In this regard, multiple heat pipes are commonly employed to tackle high flux demand. However, the wick structure in the conventional heat pipe limits the transport distance. In this regard, the concept of pulsating heat pipe (PHP) is proposed to tackle the foregoing problems (Akachi [1,2]). Unlike traditional coaxial heat pipes, the pulsating heat pipes are made from a long continuous capillary tube bent into many turns and they are free from wick. The PHPs is also easier to manufacture with fewer operating limitations [3]. Yet they required more work- ing fluid than conventional heat pipes [4]. The heat transfer of PHP occurs due to self-exciting oscillation which may be driven by fast fluctuating pressure wave engendered from nucleate boiling and subsequent condensation of the working fluid [5]. In fact, as ex- plained by Shafii et al. [5], due to the pulsation of the working fluid in the axial direction of the tube, heat is transported from the evaporator section to the condenser section. The heat input, which is the driving force, increases the pressure of the vapor plug in the evaporator section. In turn, this pressure increase will push the neighboring vapor plugs and liquid slugs toward the condenser where a lower pressure prevails. There had been quite a few studies in association with the per- formance of PHPs. Khandekar et al. [6,7] conducted experiments on a PHP made of 2 mm copper capillary tube with three different working fluids – water, ethanol and R-123. The PHP was tested in vertical (bottom heat mode) and horizontal orientation. Their results demonstrated that the effect of input power and volumetric filling ratio of the working fluid on the thermal performance is quite essential. Qu et al. [4] performed tests of a 2 mm copper cap- illary PHP having 5 turns, and ethanol was selected as the working fluid with filling ratios (FR) of 30%, 50% and 70%. Three types of attractors were identified under different power inputs. Based on their nonlinear analysis and tests, the best fill ratio is found to be at 50%. There were some studies associated the working fluids http://dx.doi.org/10.1016/j.expthermflusci.2014.01.019 0894-1777/Ó 2014 Elsevier Inc. All rights reserved. ⇑ Corresponding author. Tel.: +886 3 5712121x55105. E-mail address: ccwang@mail.nctu.edu.tw (C.-C. Wang). Experimental Thermal and Fluid Science 54 (2014) 85–92 Contents lists available at ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs