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"). From 1907 to 1909, Nusselt served as an assistant to the thermodynamicist Richard Mollier at Technische Hochschule Dresden. There, he habilitated in 1909 with the work Der Wärmeübergang in Rohrleitungen ("Heat transfer in pipes"), which laid the foundation for his later publication on dimensional analysis. His habilitation thesis included careful experiments on carbon dioxide, air, and lighting gas (Leuchtgas).

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. In 1913, he returned to Dresden, where, in February 1915, he was appointed as supernumerary professor (außerplanmäßiger Professor). Nusselt was exempted from military service in World War I for health reasons. 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 1920, Nusselt was appointed full professor (ordentlicher Professor) at Technische Hochschule Karlsruhe. 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. He was succeeded by another student of Knoblauch, Ernst Schmidt.

Scientific and technical contributions Nusselt's interests were wide-ranging, including notable mathematical analyses of conduction, forced convection, condensation and heat exchangers, as well as extensive experimental research. He established the use of nondimensional groups in correlating convection data. He identified the formal analogy between heat transfer and mass transfer. His work extended to thermal conductivity measurement, natural convection, combustion, thermal radiation, thermodynamics, gas dynamics, turbines, and internal combustion engines. Nusselt charted a new direction in research on technical thermodynamics, helping to establish engineering science (Technikwissenschaften) as a rigorous discipline. Noting that the performance of heat engines is primarily determined by the temperatures within them, he considered heat transfer processes essential to the design of such equipment. Nusselt approached thermal engineering by studying simplified components of systems, rather than entire machines, with the aim of creating understanding that would allow rational design of complex systems. Further, his use of similarity, or nondimensional groups, allowed research to conform with the fundamental conservation laws of energy, momentum, and mass without solving the governing equations.

Thermal conductivity Nusselt invented the use of a spherical shell for measuring thermal conductivity in his doctoral dissertation. The device consisted of an inner metal sphere that was electrically heated and a concentric outer metal sphere, with the test material filling the space between them. The method was attractive for its symmetry, which simplified the analysis of data, and for the absence of the edge losses inherent in planar conductivity measurements. His apparatus came to be called the Nusselt sphere, and it remained a standard technique for many decades. During his time at BASF, Nusselt extended the technique to pressurized spheres, and he continued this work with insulating powders after moving to Munich.

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