Wilhelm Nusselt

Ernst Kraft Wilhelm Nusselt (25 November 1882 – 1 September 1957) was a German mechanical engineer whose work helped establish modern heat-transfer theory. His nondimensionalization of convective heat transfer led to a widely used framework for correlating heat-transfer data, including the dimensionless group now called the Nusselt number. His theory of laminar film condensation, which identified the condensate film as the principal resistance to heat flow, yielded mathematical solutions still used today. Both results have become standard topics in heat-transfer textbooks.

Nusselt studied mechanical engineering at the Technische Hochschule München, where he received his Dipl.-Ing. in 1904 and his Dr.-Ing. in 1907 under Oskar Knoblauch. After serving as an assistant to Richard Mollier at Technische Hochschule Dresden and working briefly in industry at Sulzer and BASF, he was appointed full professor at Technische Hochschule Karlsruhe in 1920. In 1925 he moved to the chair of theoretical mechanical engineering at Technische Hochschule München, where he remained until his retirement in 1952.

Nusselt's research touched most areas of heat transfer and heat engines. He developed experimental methods for measuring thermal conductivity, analyzed cross-flow heat exchangers and falling film evaporators, and contributed to the formulation of the analogy between heat and mass transfer. He supervised about forty doctoral dissertations during his career.

Early life and education Nusselt was born on 25 November 1882 in Nuremberg, where his father was a factory owner. After completing his primary education in Nuremberg, Nusselt studied mechanical engineering at the Technische Hochschule München and at the Technische Hochschule (Berlin-)Charlottenburg, receiving his Dipl.-Ing. (engineering diploma) in 1904 from Technische Hochschule München. He then joined the Laboratory of Technical Physics at Technische Hochschule München and studied mathematics and physics while working in Oskar Knoblauch's laboratory. He obtained his Dr.-Ing. from the same institution in 1907, writing his thesis Die Wärmeleitfähigkeit von Wärmeisolierstoffen ("The thermal conductivity of thermal insulation materials").

Career Upon completing his habilitation, Nusselt became a privatdozent at Dresden, a position that he held until 1918. During this time, he worked for nine months in the heat technology division of Sulzer Brothers in Winterthur, Switzerland.

From January 1918 until March 1920, he was employed as the head of a research laboratory (Laboratoriumsvorstand) at Badische Anilin- und Soda-Fabrik (BASF) in Ludwigshafen. Beginning in 1919, he concurrently lectured as a privatdozent at Technische Hochschule Darmstadt. In 1925, he took up the chair of theoretical mechanical engineering (theoretische Maschinenlehre) at Technische Hochschule München, and he became the co-director of the Laboratory for Heat Engines (jointly with August Loschge). He held these positions until his retirement in 1952. His work extended to thermal conductivity measurement, natural convection, combustion, thermal radiation, thermodynamics, gas dynamics, turbines, and internal combustion engines. In a related paper, he performed a separation of variables analysis of thermally developing laminar flow in a tube, which exposed the variation of the heat transfer coefficient near the inlet, and he noted that this variation directly affected existing measurements of heat transfer coefficients. (Nusselt was unaware of the earlier work by the physicist Leo Graetz. This problem is sometimes called the Graetz–Nusselt problem. )

While at Dresden, Nusselt nondimensionalized the equations governing convective heat transfer, including both forced convection and natural convection, in a paper published in 1915.

The Nusselt number, now widely used in heat-transfer engineering, appeared as an unnamed group in his 1915 paper and was formally named in his honor by the Association of German Engineers (VDI) in 1931. In that paper, Nusselt also identified the groups now called the Grashof number, the Prandtl number, and the Reynolds number. In 1936, Sieder and Tate wrote that they correlated data "using dimensionless groups after the manner of Nusselt". The Nusselt number was added to American Standards Association nomenclature in 1943. By the time of his death, the term Nusselt number, abbreviated Nu, had been adopted globally. He continued to refine these correlations throughout his career, particularly as the role of the Prandtl number became more apparent.

Film condensation

Nusselt's theory applied not only to a single horizontal tube, but also to each tube in a vertical stack. His results enabled the calculation of the condensation heat transfer coefficient at each angle around each individual tube. He showed that the rate of combustion was controlled by the rate of diffusion. In a 1937 dissertation, Nusselt's student Georg Ackermann derived what is now called the Ackermann correction, which accounts for the effect of mass transfer on the heat transfer coefficient.

Other contributions Nusselt studied falling film evaporators, then gaining use in the chemical industry, first during his time at BASF, and more formally during his professorship at Karlsruhe in the early 1920s. He returned to the problem in a 1930 paper that gave a rigorous analysis of the governing differential equations, which enabled the calculation of the outlet temperature for various conditions.

At Munich, Nusselt supervised many dissertations on internal combustion engines throughout the 1930s and again after World War II. Nusselt succeeded Carl von Linde as the representative for technical thermodynamics.

Nusselt was an avid mountaineer and skier. In his memorial article, Ernst Schmidt—who succeeded Nusselt at Munich—wrote of him: "" ("Nusselt's upright character and his clear, critical nature permitted no concessions to the former regime. All outward pretense was alien to his nature.")

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